Set timeout for computing AssistData for Android assistant

Android ignores AssistData from Chrome that doesn't arrive within 3.0
seconds. The Assistant team reports that this frequently times out
with Chrome or WebView. To maximize the chances results are returned
in time, set a timeout of 2.0 seconds to get the accessibility tree
snapshot. Our underlying assumption here is that some results are
better than no results.

Note that we already had been using a limit of 5000 nodes, but the time
to populate 5000 nodes wasn't very consistent. Using a timeout will be
much more reliable.

Bug: 1161541
Change-Id: I6dfa7c37f5dc11c5a4ccd69de8533b833de00e99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2629406Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843880}

content/browser/web_contents/web_contents_android.cc

@@ -662,13 +662,16 @@ void WebContentsAndroid::RequestAccessibilitySnapshot(
   ScopedJavaGlobalRef<jobject> j_callback;
   j_callback.Reset(env, callback);
 
+  // Set a timeout of 2.0 seconds to compute the snapshot of the
+  // accessibility tree because Google Assistant ignores results that
+  // don't come back within 3.0 seconds.
   static_cast<WebContentsImpl*>(web_contents_)
       ->RequestAXTreeSnapshot(
           base::BindOnce(&AXTreeSnapshotCallback, j_callback),
           ui::kAXModeComplete,
           /* exclude_offscreen= */ false,
           /* max_nodes= */ 5000,
-          /* timeout= */ {});
+          /* timeout= */ base::TimeDelta::FromSeconds(2));
 }
 
 ScopedJavaLocalRef<jstring> WebContentsAndroid::GetEncoding(