Refactor WindowFinder definition

CL reworks the WindowFinder implementation as follows:

(Before):

- There was a WindowFinder base class, with empty ctor
and  dtor definitions, and one method GetLocalProcessWindowAtPoint,
with platform specific implementations living in
window_finder_{x11,mac,chromeos,win}.

- There used to be a derived class WindowFinderMus, overriding
GetLocalProcessWindowAtPoint.

- TabDragController instantiated either WindowFinder or
WindowFinderMus depending whether the "Mus" code path is
being executed.

(After)

- The Mac platform does not support Mus. So only
 window_finder_mac.mm and window_finder.h are built.

- Other platforms (linux_desktop, chromeos, win) build
window_finder.cc, window_finder_mus.cc as well a platform
specific file; e.g. window_finder_chromeos.cc in case of
ChromeOS.

- CL also moves the logic that checks whether chrome is
running with the Mus path to within  GetLocalProcessWindowAtPointMus.

This CL is a preparation to CL [1], where chrome --mash
capabilities are extended to run through Ozone.

[1] https://codereview.chromium.org/2408803002/

BUG=295089

Review-Url: https://codereview.chromium.org/2449103004
Cr-Commit-Position: refs/heads/master@{#429010}
7 files changed