Don't crash when calling willCheckAndDispatchMessageEvent with no proxy.
Don't crash when calling willCheckAndDispatchMessageEvent with no proxy.

It turns out to be possible to reach
RenderFrameImpl::willCheckAndDispatchMessageEvent when there is no
render_frame_proxy_.  This happens when the renderer process is about
to go away, and the top frame has a subframe that tries to send a
postMessage to the top frame in its unload handler.

BUG=475651
TEST=Crashes in the bug report go away.  Manual repro: navigate to any
of the URLs in the crash report (e.g.,
http://jsatt.com/blog/class-based-celery-tasks/).  Navigate the tab
cross-site to force a process swap and make sure the renderer doesn't
crash.

Review URL: https://codereview.chromium.org/1073963003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#324565}

hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,ltilve/chromium,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,ltilve/chromium,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,ltilve/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,PeterWangIntel/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,ltilve/chromium,axinging/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk