Re-disable broken page in key_silk_cases.

The following page always errors out inside rasterize_and_record.key_silk_cases
on some windows platforms:
http://plus.google.com/app/basic/stream

This change disables this page so that the perf bots can pass this test for the
other pages.  The test was previously disabled, but then got accidentally
re-enabled as part of
http://src.chromium.org/viewvc/chrome?view=revision&revision=254357

BUG=338838
TBR=dominikg@chromium.org,tonyg@chromium.org

Review URL: https://codereview.chromium.org/181013013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254970 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254970