BrowserSwitcherService: split implementation between platforms

This CL adds BrowserSwitcherServiceWin, which extends
BrowserSwitcherService and contains the Windows-specific code. This
avoids too many #ifdefs in that file.

Change-Id: Icbf49f82e605710c273c8795805c4ad962b67ea7
Reviewed-on: https://chromium-review.googlesource.com/c/1457221
Reviewed-by: Julian Pastarmov <[email protected]>
Commit-Queue: Nicolas Ouellet-Payeur <[email protected]>
Cr-Commit-Position: refs/heads/master@{#629945}
7 files changed