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This happens when the WebContents being destroyed. If the
InterstitialPage is showing we only clear the reference
to the native WebContentsAccessibility from InterstitialPage's
BrowserAccessibilityManager, use-after-free will be happened
when the |WebContents->GetMainFrame|'s BrowserAccessibilityManager
destroyed.

Change RawPtr of WebContentsAccessibilityAndroid to WeakPtr.

Bug:1056882

Change-Id: Id1efea84aef6c7437a178f7006bc09035949bbcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063789
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746202}