[Cleanup] rename WebContentsModalDialogManager::IsShowingDialog() to IsDialogActive()

The function tests whether web contents modal dialogs are active, rather
than if they are actually showing. This updates the name accordingly
and resolves the TODO.

BUG=157161

Review URL: https://chromiumcodereview.appspot.com/23815006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222289 0039d316-1c4b-4281-b951-d872f2087c98
17 files changed