Fix potential flake in PrerenderBrowserTest.PrerenderPageNewTab.
Fix potential flake in PrerenderBrowserTest.PrerenderPageNewTab.

When the swap starts, about:blank hasn't finished loading yet. If the merge
takes too long, the ProvisionalChangeToMainFrameUrl from the previous
navigation may creep in and we incorrectly cancel the pending swap. This race
isn't a problem for correctness since it is always safe to cancel the pending
swap, though it is a case where we don't swap when we potentially could in the
event of a very very tight race occurring. To avoid it flaking the test, wait
for about:blank in the new tab to load first.

Triggered flake by increasing the merge timeout and putting a sleep in
DOMStorageNamespace::Merge.

BUG=none
TEST=PrerenderBrowserTest.PrerenderPageNewTab

Review URL: https://codereview.chromium.org/115263003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@240759 0039d316-1c4b-4281-b951-d872f2087c98

Jonekee/chromium.src,M4sse/chromium.src,anirudhSK/chromium,markYoungH/chromium.src,M4sse/chromium.src,bright-sparks/chromium-spacewalk,Chilledheart/chromium,Just-D/chromium-1,patrickm/chromium.src,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,jaruba/chromium.src,Jonekee/chromium.src,krieger-od/nwjs_chromium.src,M4sse/chromium.src,M4sse/chromium.src,hgl888/chromium-crosswalk-efl,krieger-od/nwjs_chromium.src,Jonekee/chromium.src,crosswalk-project/chromium-crosswalk-efl,dednal/chromium.src,dushu1203/chromium.src,dednal/chromium.src,mohamed--abdel-maksoud/chromium.src,jaruba/chromium.src,ondra-novak/chromium.src,fujunwei/chromium-crosswalk,Jonekee/chromium.src,dushu1203/chromium.src,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk-efl,anirudhSK/chromium,ChromiumWebApps/chromium,dushu1203/chromium.src,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,markYoungH/chromium.src,mohamed--abdel-maksoud/chromium.src,mohamed--abdel-maksoud/chromium.src,chuan9/chromium-crosswalk,Just-D/chromium-1,dednal/chromium.src,krieger-od/nwjs_chromium.src,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,markYoungH/chromium.src,Chilledheart/chromium,chuan9/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,axinging/chromium-crosswalk,Just-D/chromium-1,bright-sparks/chromium-spacewalk,anirudhSK/chromium,krieger-od/nwjs_chromium.src,Fireblend/chromium-crosswalk,Chilledheart/chromium,M4sse/chromium.src,bright-sparks/chromium-spacewalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk-efl,TheTypoMaster/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,krieger-od/nwjs_chromium.src,PeterWangIntel/chromium-crosswalk,bright-sparks/chromium-spacewalk,axinging/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,dushu1203/chromium.src,mohamed--abdel-maksoud/chromium.src,PeterWangIntel/chromium-crosswalk,Jonekee/chromium.src,ltilve/chromium,ChromiumWebApps/chromium,dushu1203/chromium.src,jaruba/chromium.src,M4sse/chromium.src,ChromiumWebApps/chromium,krieger-od/nwjs_chromium.src,crosswalk-project/chromium-crosswalk-efl,ChromiumWebApps/chromium,anirudhSK/chromium,Chilledheart/chromium,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,ondra-novak/chromium.src,littlstar/chromium.src,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,jaruba/chromium.src,PeterWangIntel/chromium-crosswalk,M4sse/chromium.src,ondra-novak/chromium.src,markYoungH/chromium.src,ltilve/chromium,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,bright-sparks/chromium-spacewalk,markYoungH/chromium.src,Fireblend/chromium-crosswalk,littlstar/chromium.src,Chilledheart/chromium,patrickm/chromium.src,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,jaruba/chromium.src,Fireblend/chromium-crosswalk,ChromiumWebApps/chromium,ChromiumWebApps/chromium,krieger-od/nwjs_chromium.src,crosswalk-project/chromium-crosswalk-efl,mohamed--abdel-maksoud/chromium.src,ChromiumWebApps/chromium,markYoungH/chromium.src,PeterWangIntel/chromium-crosswalk,jaruba/chromium.src,dushu1203/chromium.src,markYoungH/chromium.src,markYoungH/chromium.src,dednal/chromium.src,hgl888/chromium-crosswalk,anirudhSK/chromium,dushu1203/chromium.src,Pluto-tv/chromium-crosswalk,littlstar/chromium.src,dednal/chromium.src,ondra-novak/chromium.src,jaruba/chromium.src,patrickm/chromium.src,hgl888/chromium-crosswalk-efl,patrickm/chromium.src,markYoungH/chromium.src,krieger-od/nwjs_chromium.src,jaruba/chromium.src,ltilve/chromium,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,krieger-od/nwjs_chromium.src,hgl888/chromium-crosswalk-efl,M4sse/chromium.src,anirudhSK/chromium,ondra-novak/chromium.src,crosswalk-project/chromium-crosswalk-efl,mohamed--abdel-maksoud/chromium.src,Chilledheart/chromium,hgl888/chromium-crosswalk,jaruba/chromium.src,Jonekee/chromium.src,fujunwei/chromium-crosswalk,ChromiumWebApps/chromium,ltilve/chromium,chuan9/chromium-crosswalk,bright-sparks/chromium-spacewalk,M4sse/chromium.src,Fireblend/chromium-crosswalk,M4sse/chromium.src,Fireblend/chromium-crosswalk,Just-D/chromium-1,jaruba/chromium.src,Pluto-tv/chromium-crosswalk,patrickm/chromium.src,M4sse/chromium.src,dednal/chromium.src,anirudhSK/chromium,Fireblend/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,dednal/chromium.src,Just-D/chromium-1,ltilve/chromium,crosswalk-project/chromium-crosswalk-efl,ChromiumWebApps/chromium,ltilve/chromium,TheTypoMaster/chromium-crosswalk,littlstar/chromium.src,patrickm/chromium.src,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk-efl,ltilve/chromium,anirudhSK/chromium,dednal/chromium.src,PeterWangIntel/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,Jonekee/chromium.src,anirudhSK/chromium,bright-sparks/chromium-spacewalk,TheTypoMaster/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,PeterWangIntel/chromium-crosswalk,dushu1203/chromium.src,hgl888/chromium-crosswalk-efl,Pluto-tv/c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