[bedrock] Update CloseAllSecondaryWidgets() to CloseAllWidgets()

This CL updates Widget::CloseAllSecondaryWidgets() to be
CloseAllWidgets(), which ensures all platform Widgets are closed, not
just secondary Widgets.

Currently Widget::CloseAllSecondaryWidgets() is only called by
HandleAppExitingForPlatform(). At this point it is assumed that all
Browsers have been closed / destroyed. The intent is to ensure all
platform Widgets have been destroyed before the app exits.

This currently works on the implicit assumption that Browsers are owned
by their platform Widget using the NATIVE_WIDGET_OWNS_WIDGET scheme -
and hence all Browser platform Widgets have been destroyed at the point
HandleAppExitingForPlatform() is invoked.

However under CLIENT_OWNS_WIDGET the Browser may have been destroyed but
their platform Widgets remain, which will eventually be cleaned up async
by the host platform.

By instead ensuring all platform Widgets are closed in
HandleAppExitingForPlatform(), this CL eliminates the implicit
assumptions on the Widget ownership scheme. It is also safe to close
Browser platform Widgets synchronously since lifetimes between Widget
and NativeWidgets have already been decoupled.

Bug: 413168662
Include-Ci-Only-Tests: true
Change-Id: I24ad0f7ea010f3b1413a65e8f6209e993d24ccce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6820476
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Tom Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1498412}