[firefox upgrade] Replace action chain with tinymce JS.

This action chain is not reliable across firefox versions (and
in fact, even on the older firefox, fails locally). Instead we
use native JS tinymce to set the content.

common/test/acceptance/pages/studio/html_component_editor.py

@@ -39,18 +39,13 @@ class HtmlComponentEditorView(ComponentEditorView):
         self.cancel()
 
     def set_content(self, content):
-        """Types content into the html component, leaving the component open.
+        """Sets content in the html component, leaving the component open.
 
         Arguments:
             content (str): The content to be used.
         """
         self.q(css=self.editor_mode_css).click()
-
-        selector = '.html-editor .mce-edit-area'
-        editor = self.q(css=self._bounded_selector(selector))[0]
-        ActionChains(self.browser).click(editor).\
-            send_keys([Keys.CONTROL, 'a']).key_up(Keys.CONTROL).\
-            send_keys(content).perform()
+        self.browser.execute_script("tinyMCE.activeEditor.setContent('%s')" % content)
 
     def set_raw_content(self, content):
         """Types content in raw html mode, leaving the component open.