Force child iframe to be focused.

This test sometimes fails because the child iframe containing the
text area isn't focused when we check at the start of the test. This
CL forces it to be focused.

Bug: 1024672
Change-Id: I14c04823bee5d6e04be9cd4c5d0848bf31247dba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919325Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715781}

chrome/browser/pdf/pdf_extension_test.cc

@@ -449,7 +449,11 @@ IN_PROC_BROWSER_TEST_F(PDFExtensionTestWithTestGuestViewManager,
                       "});")
           .ExtractBool());
 
-  // Make sure the text area is focused.
+  // Make sure the text area is focused. First, we must explicitly focus the
+  // child iframe containing the text area.
+  content::RenderFrameHost* main_frame = embedder_web_contents->GetMainFrame();
+  content::RenderFrameHost* child_text_area = ChildFrameAt(main_frame, 0);
+  ASSERT_TRUE(content::ExecJs(child_text_area, "window.focus();"));
   ASSERT_TRUE(content::EvalJs(
                   embedder_web_contents,
                   "new Promise((resolve) => {"