PerFrameTranslate: Posts the OnPageTranslated callback on thread's queue

This is an attempt to avoid a race condition crash with the infobar
going away while the page translated callback occurs.

Bug: 1093320
Change-Id: I47025de0d8c23045e1a964d4cef0c732e903b8b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283657Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785850}

components/translate/content/browser/per_frame_content_translate_driver.cc

@@ -453,8 +453,13 @@ void PerFrameContentTranslateDriver::OnFrameTranslated(
   }
 
   if (--stats_.pending_request_count == 0) {
-    OnPageTranslated(cancelled, original_lang, translated_lang,
-                     stats_.main_frame_error);
+    // Post the callback on the thread's task runner in case the
+    // info bar is in the process of going away.
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::BindOnce(&ContentTranslateDriver::OnPageTranslated,
+                                  weak_pointer_factory_.GetWeakPtr(), cancelled,
+                                  original_lang, translated_lang,
+                                  stats_.main_frame_error));
     stats_.Report();
     stats_.Clear();
   }