Support automated iFrame Processing

[huaxing-yuan]

Issue and Steps to Reproduce

iFrame is a big barrier to web test automation:

• Sometimes it's hard for users to "aware" whether the element is in an iFrame.
• Switching the context between main page and iFrames are also hard to manage.

Proposition

• WebEngine Framework would review object identification strategy, and can automatically find elements within the iFrames if possible.
• The Framework handles context automatically.
• Users can just write test scripts straightly.