Add LayoutTest for uncancelable TouchEvents

Now that the necessary EventSender changes have landed in chromium
(https://src.chromium.org/viewvc/chrome?view=rev&revision=265991),
I can more completely test my change for uncancelable TouchEvents
(http://src.chromium.org/viewvc/blink?view=revision&revision=172329).

BUG=365681

Review URL: https://codereview.chromium.org/250843005

git-svn-id: svn://svn.chromium.org/blink/trunk@172689 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unused RenderBlock::regionAtBlockOffset()

Review URL: https://codereview.chromium.org/250913002

git-svn-id: svn://svn.chromium.org/blink/trunk@172690 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce ScriptForbiddenScope and guard updateStyle, performLayout and compositing updates

To prevent more cases of script running inside these processes this patch
introduces a scope object that ASSERTS if we try to call into V8 from them.

The only catch is <svg> can initialize the DOMWindow from inside of recalcStyle,
so we also add a special case with AllowScriptForDOMWindowInitialize that
disables the ASSERTS for DOMWindow::initialize. In the future we might make
that async instead.

Note that this scope doesn't use the DocumentLifecycle since not all entry
points to V8 in V8ScriptRunner have access to the ExecutionContext. It also
allows us to put this scope in other places in the code base where lifecycle
might technically allow script execution, but we wish to forbid it for that
function.

Review URL: https://codereview.chromium.org/250803002

git-svn-id: svn://svn.chromium.org/blink/trunk@172691 bbb929c8-8fbe-4397-9dbb-9b2b20218538