Don't disable non-fast-scroll region tracking when touch is disabled.

r154611 included an optimization which disabled touch hit rect tracking when touch events were disabled.  I also disabled the 'shouldHandleScrollGestureOnMainThreadRegion' tracking at the same time since scroll gestures can only be produced by touch.  But it turns out this code is also used for mousewheel events (it's really 'not fast-scroll region tracking').

This also addresses flakiness in   compositing/iframes/iframe-content-flipping.html after this change.  In particular, the call to WebLayer::setBounds is also required even when touch events are disabled.

Note that there's currently no good way to test this change in a LayoutTest because content_shell always enables touch event support and we don't have any mechanism for layout tests to run with different flags (touch support can't be changed once v8 has started).  Also the existing scrollingcoordinator tests call computeShouldHAndleScrollGestureOnMainThreadRegion directly rather than relying on this call in frameViewLayoutUpdated.  Instead fsamuel@ plans to submit a browser test in the chrome tree for this from the webview perspective.

BUG=263266,263120

Review URL: https://chromiumcodereview.appspot.com/19814004

git-svn-id: svn://svn.chromium.org/blink/trunk@154780 bbb929c8-8fbe-4397-9dbb-9b2b20218538