base: Increase emulated discardable memory limit.

Google Play Music use an image that is 34MB decoded. This image is
somehow used to raster every single tile produced by impl-side
painting and it not fitting into the cache causes raster performance
to plummet.

This increases the limit to 64MB but also makes us discard more memory
under moderate pressure so the total amount of memory used by chrome
is not significantly increased.

BUG=341323

Review URL: https://codereview.chromium.org/151483011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249983 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249983