|WebFrameImpl::fromFrame| should check type before FrameLoaderClientImpl downcast.

There are two possible FrameLoaderClient implementation, FrameLoaderClientImpl, and EmptyFrameLoaderClient.
This CL adds type query isFrameLoaderClientImpl before performing downcast.

BUG=350686

Review URL: https://codereview.chromium.org/196503003

git-svn-id: svn://svn.chromium.org/blink/trunk@169001 bbb929c8-8fbe-4397-9dbb-9b2b20218538