GitLab

The main user of GLib MessagePump implementation is Aura/X11, while Ozone-based
ports use mainly libevent version, which exposes WatchFileDescriptor API that
is used by higher level components through MessageLoop[Current]For{UI,IO}
interfaces. Currently, there's no such API in MessagePumpGlib.

The motivation here is to make MessageLoopCurrent API usage transparent,
regardless which message pump is used in UI thread. This benefits X11 migration
to Ozone on Linux desktop, which (just like Aura/X11) must integrate with Gtk,
so GLib-based loop in browser's main thread is a natural path.

So, this CL implements the aforementioned API in GLib MessageLoop following the
same design, assumptions and overall structure as its original libevent-based
version.

As a follow-up, https://crrev.com/c/1730972 refactors code using
MessagePumpLibevent::{FdWatchController,FdWatcher} directly, replacing them by
MessageLoopCurrent*ForUI counterparts as well as refactoring
X11EventSourceLibevent (which Implementation has been used as reference for
this CL) in the same direction, so in the end it has no libevent specific code
and the build-time glib/libevent switch (use_glib gn var) is possible.

Bug: 988094, 789065
Test: base_unittests (MessagePumpGLibFdWatchTest.*)
Change-Id: I843ab924d18c08b6384e0716bc36c4f5f8482fa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731031
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685245}