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Safe browsing webview tests were flaky with committed interstitials
enabled caused by a lack of a reliable way to wait for an interstitial
to show. This CL exposes IsShowingInterstitial from security_
interstitial_tab_helper.h and changes the tests to poll it before
waiting for the visual state callback.

Bug: 1046832
Change-Id: If82f7fbbd9896e4cc3611e48fe945c8581cd847a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027902
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736528}