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This is a follow-up to http://crrev/c/958229.

This moves the implementation of getSafeBrowsingPrivacyPolicyUrl() to
the UI thread, and adds a DCHECK to assert this. This was done under
boliu@'s recommendation, since AwBrowserContext seems like it should be
used mainly on the UI thread only (based on how content::BrowserContext
is used).

We still permit apps to call WebView#getSafeBrowsingPrivacyPolicyUrl()
on any thread, since it is a static method. We accommodate this by
blocking and running on the UI thread.

Test: run_webview_instrumentation_test_apk -f SafeBrowsingTest#testGetSafeBrowsingPrivacyPolicyUrl
Change-Id: I275825d6a49e08247e2839ec4a93272ce4204c98
Reviewed-on: https://chromium-review.googlesource.com/958040Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542901}