Change "high RAM" level for iOS devices from 250 MB to 500MB.

All supported devices have at least 256MB so this change aims to keep the method relevant.

BUG=327736

Review URL: https://codereview.chromium.org/99473011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240194 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240194