To make the HeadlessCompositorWidget work under Windows as well as Linux, I had
to change the way that I hooked it into the existing CompositorWidget system.
Under GTK, the CompositorWidgetInitData and CompositorWidgetDelegate types
provided the information needed by the headless compositor widget already (the
widget client size). On Windows, however, the definitions of these types
differ, and the client size information is simply retrieved from the platform
APIs when needed.
After this patch, CompositorWidgetDelegate is renamed to
PlatformCompositorWidgetDelegate, and a new base class called
CompositorWidgetDelegate is added with "AsPlatformSpecificDelegate()" and
"AsHeadlessCompositorWidget()" methods. In non-headless mode, widgets use
AsPlatformSpecificDelegate() to access the Windows- and GTK-specific delegate
APIs. In headless mode, AsHeadlessCompositorWidget() is used to access the
singular CompositorWidget implementation for headless. Meanwhile, the
CompositorWidgetInitData IPDL type is made into a union which always contains a
headless-specific HeadlessCompositorWidgetInitData struct and under GTK and
Windows also contains an {X11,Win}CompositorWidgetInitData struct.
This also includes a small patch to ensure that the GPU process and
hardware-accelerated compositing are always disabled under headless mode. These
features weren't activated by default in the Linux environments I tested in, but
did end up activating (and then promptly crashing Firefox) when I tested on
Windows.
MozReview-Commit-ID: CocPoHBDV7H
