Add WebContentsModalDialogManager unit tests

Implement additional tests covering most of the functionality of
WebContentsModalDialogManager. Refactor common setup into the test
fixture.

BUG=157161
R=sky@chromium.org

Review URL: https://codereview.chromium.org/23475022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221483 0039d316-1c4b-4281-b951-d872f2087c98

components/web_modal/web_contents_modal_dialog_manager.h

@@ -38,7 +38,9 @@ class WebContentsModalDialogManager
   // WillClose() when it is being destroyed.
   void ShowDialog(NativeWebContentsModalDialog dialog);
 
-  // Returns true if a dialog is currently being shown.
+  // Returns true if any dialogs are active and not closed.
+  // TODO(wittman): Give this a better name, since it can return true if no
+  // dialogs are displayed.
   bool IsShowingDialog() const;
 
   // Focus the topmost modal dialog.  IsShowingDialog() must be true when