Fix a long standing bug with the browser window losing focus when the user scrolled...

Fix a long standing bug with the browser window losing focus when the user scrolled on the page using certain trackpad drivers.

The bug occurs because the browser window receives a WM_MOUSEACTIVATE message when the trackpad is
operated on the web page. There is code in the HWNDMessageHandler::OnMouseActivate function which
checks if the window under the cursor is a child window and if yes it causes the focus manager
to lose activation. This code was added to support activation in windowed NPAPI plugins.

It causes this issue because the child window in question here is the legacy RenderWidgetHostHWND
class.

Fix for now is to ignore transparent child windows. Added a TODO in HWNDMessageHandler to remove the
child window related code once we deprecate NPAPI plugins.

BUG=351995

Review URL: https://codereview.chromium.org/626183002

Cr-Commit-Position: refs/heads/master@{#298151}