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This modality can be used in tests rather than choosing
an arbitrary modality for the test, reducing the coupling
between overlay functionality and specific implementations
for the web content area or infobars.

This CL adds support for contained and presented overlay
UI to test overlay UI code.  A follow-up CL will add
tests to OverlayPresentationContextImpl that leverages these
classes.

Change-Id: I648f95a6af1fc78c007fb8d96a7c6ca295928bae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068254
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746513}