Set compositor to only allow "nice to have" textures with software renderer.

The default memory management setting of allowing everything was keeping a bunch of marginally-useful bitmaps around, so we should cut down the limit so only "nice to have" bitmaps are kept around.

BUG=293933

Review URL: https://chromiumcodereview.appspot.com/24357004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224943 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224943
Fix name for kDisableWebKitMediaSource in about://flags.

TBR=acolwell@chromium.org
BUG=none

Review URL: https://chromiumcodereview.appspot.com/24360007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224944 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224944