Allow passing mock FrameLoaderClient to DummyPageHolder.

This CL has three parts:
- DummyPageHolder can be created with a given FrameLoaderClient instance.
- EmptyFrameLoaderClient is no longer FINAL so it can be used for mocks.
- fix dependency breakage following the move of EmptyClients.h in the cpp.

BUG=None

Review URL: https://codereview.chromium.org/467943005

git-svn-id: svn://svn.chromium.org/blink/trunk@181312 bbb929c8-8fbe-4397-9dbb-9b2b20218538