AW: mark variable as synchronized

This marks Safe Browsing user opt-out as volatile, since it may be
set by a background thread.

This also documents that querySafeBrowsingUserConsent can invoke the
callback on any thread (because this will be implemented by
http://crrev/i/615168).

Bug: 835497
Test: manual
Change-Id: I557dd5979aeaf7a4a2119ec0c8604528278e05a6
Reviewed-on: https://chromium-review.googlesource.com/1029423
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554118}

android_webview/java/src/org/chromium/android_webview/AwSafeBrowsingConfigHelper.java

@@ -26,7 +26,7 @@ public class AwSafeBrowsingConfigHelper {
     private static final String OPT_IN_META_DATA_STR = "android.webkit.WebView.EnableSafeBrowsing";
     private static final boolean DEFAULT_USER_OPT_IN = false;
 
-    private static Boolean sSafeBrowsingUserOptIn;
+    private static volatile Boolean sSafeBrowsingUserOptIn;
 
     // Used to record the UMA histogram SafeBrowsing.WebView.AppOptIn. Since these values are
     // persisted to logs, they should never be renumbered nor reused.

android_webview/java/src/org/chromium/android_webview/PlatformServiceBridge.java

@@ -51,6 +51,7 @@ public boolean canUseGms() {
         return false;
     }
 
+    // Overriding implementations may call "callback" asynchronously, on any thread.
     public void querySafeBrowsingUserConsent(
             Context context, @NonNull final Callback<Boolean> callback) {
         // User opt-in preference depends on a SafetyNet API.