Make TopSitesTest unit_test more robust.
Make TopSitesTest unit_test more robust.

The number of prepopulated URLs on the top sites page
can vary depending on configuration. Change the unit test
to query this value instead of hardcoding it.

BUG=None
TEST=TopSitesTest.*


Review URL: http://codereview.chromium.org/8384032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@120384 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,ropik/chromium