Add Platform::physicalMemoryInBytes to WebKitPlatformSupportImpl.

BUG=312241

Review URL: https://codereview.chromium.org/50903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233005 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233005