Route 'Select All' OS method through RenderWidgetHostDelegate.

This allows Cmd-A / "Select All" to be used on SSL interstitials on OSX.

BUG=507416

Review URL: https://codereview.chromium.org/1212373009

Cr-Commit-Position: refs/heads/master@{#338248}
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 0ebb78b..3ef5847e 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -567,10 +567,11 @@
   BrowserAccessibilityManager* GetRootBrowserAccessibilityManager() override;
   BrowserAccessibilityManager* GetOrCreateRootBrowserAccessibilityManager()
       override;
-  // Following three functions are already listed under WebContents overrides.
+  // The following 4 functions are already listed under WebContents overrides:
   // void Cut() override;
   // void Copy() override;
   // void Paste() override;
+  // void SelectAll() override;
   void MoveRangeSelectionExtent(const gfx::Point& extent) override;
   void SelectRange(const gfx::Point& base, const gfx::Point& extent) override;