[XProto] Reintroduce connection flushes after PutImage requests

These were removed in [1] because the message loop was changed to
TYPE_UI, which automatically flushes.  However, if there are a lot of
tasks in the queue, the flush may happen later than we'd like, and
this increases latency between frames.  This CL adds the flushes back
in an effort to fix perf regressions.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2343314

R=sky
BUG=1119639

Change-Id: I04d6854fce7df69830f421dd81d734a14ae6d5a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377609
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802363}

ui/base/x/x11_software_bitmap_presenter.cc

@@ -16,7 +16,6 @@
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted_memory.h"
-#include "base/task/current_thread.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkImageInfo.h"
 #include "third_party/skia/include/core/SkSurface.h"
@@ -218,6 +217,9 @@ void X11SoftwareBitmapPresenter::EndPaint(const gfx::Rect& damage_rect) {
         .offset = 0,
     });
     needs_swap_ = true;
+    // Flush now to ensure the X server gets the request as early as
+    // possible to reduce frame-to-frame latency.
+    connection_->Flush();
     return;
   }
   if (surface_)
@@ -229,6 +231,10 @@ void X11SoftwareBitmapPresenter::EndPaint(const gfx::Rect& damage_rect) {
   if (composite_ &&
       CompositeBitmap(connection_, widget_, rect.x(), rect.y(), rect.width(),
                       rect.height(), depth_, gc_, skia_pixmap.addr())) {
+    // Flush now to ensure the X server gets the request as early as
+    // possible to reduce frame-to-frame latency.
+
+    connection_->Flush();
     return;
   }
 
@@ -236,8 +242,9 @@ void X11SoftwareBitmapPresenter::EndPaint(const gfx::Rect& damage_rect) {
   DrawPixmap(connection, visual_, widget_, gc_, skia_pixmap, rect.x(), rect.y(),
              rect.x(), rect.y(), rect.width(), rect.height());
 
-  // We must be running on a UI thread so that the connection will be flushed.
-  DCHECK(base::CurrentUIThread::IsSet());
+  // Flush now to ensure the X server gets the request as early as
+  // possible to reduce frame-to-frame latency.
+  connection_->Flush();
 }
 
 void X11SoftwareBitmapPresenter::OnSwapBuffers(