[Chromecast] Reuse the Aura window manager across receiver apps.

Always makes the root window transparent for seeing the video plane
beneath it.

Allows tearing down and rebuilding the window manager.

BUG=internal b/34103918, internal b/33835219, internal b/33046596
TEST=manually test all categories of receiver apps and mirroring.

Change-Id: Ic84079e6eae012896b2a1d8bdf39dc64578f2307
Review-Url: https://codereview.chromium.org/2643553002
Cr-Commit-Position: refs/heads/master@{#445424}

• chromecast/browser/BUILD.gn[diff]
• chromecast/browser/DEPS[diff]
• chromecast/browser/android/cast_content_window_android.cc[diff]
• chromecast/browser/android/cast_content_window_android.h[diff]
• chromecast/browser/cast_browser_main_parts.cc[diff]
• chromecast/browser/cast_browser_main_parts.h[diff]
• chromecast/browser/cast_content_browser_client.cc[diff]
• chromecast/browser/cast_content_browser_client.h[diff]
• chromecast/browser/cast_content_window.h[diff]
• chromecast/browser/cast_content_window_linux.cc[diff]
• chromecast/browser/cast_content_window_linux.h[diff]
• chromecast/browser/service/cast_service_simple.cc[diff]
• chromecast/browser/service/cast_service_simple.h[diff]
• chromecast/browser/test/cast_browser_test.cc[diff]
• chromecast/graphics/BUILD.gn[diff]
• chromecast/graphics/DEPS[diff]
• chromecast/graphics/cast_window_manager.h[Added - diff]
• chromecast/graphics/cast_window_manager_aura.cc[Added - diff]
• chromecast/graphics/cast_window_manager_aura.h[Added - diff]
• chromecast/graphics/cast_window_manager_default.cc[Added - diff]
• chromecast/graphics/cast_window_manager_default.h[Added - diff]