Add --enable-html-imports commandline switch and about-flags entry.

BUG=240592
R=dglazkov,ojan,creis

Review URL: https://chromiumcodereview.appspot.com/22342005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@216462 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed