Touch event flag should control only DOM event firing.

Currently there is no way to disable TouchEvent API support
without affecting either PointerEvents (for touches) or
touch support in browser UI. With PointerEvents shipping in
M55, it now makes the most sense to turn on touch support in
browser unconditionally and let the --touch-events flag
control only the firing of DOM events.

This CL also renames the flag to clarify its purpose.

BUG=644318

Review-Url: https://codereview.chromium.org/2467913002
Cr-Commit-Position: refs/heads/master@{#434399}
28 files changed