Rename the "Experimental WebKit Features" flag.

Since we're not WebKit-based any more, this flag is confusingly named.
"Web Platform" more accurately reflects the flag's intent.

BUG=238153

Review URL: https://chromiumcodereview.appspot.com/14940009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@210820 0039d316-1c4b-4281-b951-d872f2087c98