webkit: Avoid use of discardable memory unless supported natively.

BUG=237681

Review URL: https://codereview.chromium.org/60273004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233187 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233187