Separate the scrolling code out of RenderLayer

RenderLayer was a ScrollableArea even though it couldn't scroll (e.g.
if it had overflow: visible). This change is the mechanical addition
of RenderLayerScrollableArea. For now, the new class is a shim that
forwards all calls to RenderLayer but more logic will be moved to
the class in follow-up patches.

The current logic checks the style for allocating the ScrollableArea,
which is the safest but could end up allocating more than needed
(e.g. for overflow: auto).

BUG=260899
R=abarth@chromium.org, esprehn@chromium.org, jamesr@chromium.org, vollick@chromium.org

Review URL: https://codereview.chromium.org/19218007

git-svn-id: svn://svn.chromium.org/blink/trunk@154778 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove the capacity to cancel a load via nulling the ResourceRequest inside dispatchWillSendRequest().

Also, remove associated tests.

This capacity is only used in content_shell.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19741005

git-svn-id: svn://svn.chromium.org/blink/trunk@154779 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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
Move createPlugin from WebTestProxy to WebFrameTestProxy.

BUG=245126

Review URL: https://chromiumcodereview.appspot.com/19965005

git-svn-id: svn://svn.chromium.org/blink/trunk@154781 bbb929c8-8fbe-4397-9dbb-9b2b20218538