Move webkit/child/web_discardable_memory_impl to content/child/

BUG=338338

Review URL: https://codereview.chromium.org/130713007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248057 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248057