Fix chromeos-chrome resize problem on glinux

Bug: 1147605
Change-Id: I3aead63cdb84b4bc31b992194de22244d6c0242b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570030Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833908}

ui/gl/gl_surface_glx.cc

@@ -106,6 +106,7 @@ class OMLSyncControlVSyncProvider : public SyncControlVSyncProvider {
   bool GetSyncValues(int64_t* system_time,
                      int64_t* media_stream_counter,
                      int64_t* swap_buffer_counter) override {
+    x11::Connection::Get()->Flush();
     return glXGetSyncValuesOML(x11::Connection::Get()->GetXlibDisplay(),
                                glx_window_, system_time, media_stream_counter,
                                swap_buffer_counter);