AW: move getSafeBrowsingPrivacyPolicyUrl to UI thread

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}

android_webview/browser/aw_contents_statics.cc

@@ -58,7 +58,7 @@ ScopedJavaLocalRef<jstring>
 JNI_AwContentsStatics_GetSafeBrowsingPrivacyPolicyUrl(
     JNIEnv* env,
     const JavaParamRef<jclass>&) {
-  // TODO(ntfschr): limit this to only the UI thread.
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
   GURL privacy_policy_url(
       security_interstitials::kSafeBrowsingPrivacyPolicyUrl);
   std::string locale =

android_webview/glue/java/src/com/android/webview/chromium/SharedStatics.java

@@ -110,6 +110,7 @@ public class SharedStatics {
      * to users.
      */
     public Uri getSafeBrowsingPrivacyPolicyUrl() {
-        return AwContentsStatics.getSafeBrowsingPrivacyPolicyUrl();
+        return ThreadUtils.runOnUiThreadBlockingNoException(
+                () -> AwContentsStatics.getSafeBrowsingPrivacyPolicyUrl());
     }
 }