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Currently TouchAccessibilityBrowserTest.TouchExplorationInCrossSiteIframe is
calling WaitForChildFrameSurfaceReady. However it is not actually waiting, as
the frame it is providing is not a RenderWidgetHostViewChildFrame.

This change updates the call to the new hit testing api HitTestRegionObserver.
This will allow the test to properly wait on hit testing data while run under
Viz. While maintaining the current early exit for non-Viz.

TEST=TouchAccessibilityBrowserTest.TouchExplorationInCrossSiteIframe

Bug: 787945
Change-Id: Ica7fa92e7dcb5ea3d7a5b9702f9c5d077d929c07
Reviewed-on: https://chromium-review.googlesource.com/1102570Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568054}