Only BrowserWindowController implements this. Other implementors just
turn it into a no-op. ChromeEventProcessingWindow (CEPW) now simply
checks whether it's a Browser window and executes the command.

This is the first step to factor out keyboard shortcut handling in CEPW into
a component that can be reused in Views as it removes one (of two)
dependencies on -[NSWindow windowController], which cannot be used
to implement browser-side behavior under Views.

Added new browser_test for shortcuts in global_keyboard_shortcuts_mac.

Some history:
BrowserCommandExecutor was added in r30619 (6 years ago) to allow a
non-browser window to execute browser commands, but became obsolete
a month later in r32445 when HtmlDialogWindowController (now
WebDialogWindowController) decided to ignore those commands instead.

BUG=508438

Review URL: https://codereview.chromium.org/1250533003

Cr-Commit-Position: refs/heads/master@{#341062}