Use validating IPC enum macros in cast_messages.h

This is part of a long-running background task to remove the remaining
uses of the unchecked IPC_ENUM_TRAITS() macro.

BUG=246708

Review URL: https://codereview.chromium.org/170263007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252264 0039d316-1c4b-4281-b951-d872f2087c98
Remove --enable-input-mode-attribute flag

Enable the "inputmode" HTML5 attribute via Blink's
experimental feature flag.

BUG=244688
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/170293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252265 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252264
SVN changes up to revision 252265