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We've been seeing in the histograms that for some users, page feature
extraction is taking much longer than we would like, up to 200ms between
stops. This could possibly contribute to jankiness in the UI. These are some
timings from my desktop for how long it takes to extract features before and
after these changes. Obviously this is on much better hardware than the use case
we are worried about, but hopefully the increase is speed is proportional.

           Before  After
CNN        19       7.5    
ESPN       22       9.5
Google     12       4
Salon      40       12

BUG=
TEST=Ran associated unit tests.


Review URL: http://codereview.chromium.org/7549003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96097 0039d316-1c4b-4281-b951-d872f2087c98