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In OnMemoryPressure, drop tiles on invisible browser views.

Not reusing base::MemoryPressureListener because Android
WebView requires onTrimMemory to be called synchronously.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/25082006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226643 0039d316-1c4b-4281-b951-d872f2087c98