gl: Disable SGI_video_sync by default.

ui/gfx/vsync_provider.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_VSYNC_PROVIDER_H_
 #define UI_GFX_VSYNC_PROVIDER_H_
 
 #include "base/callback.h"
 #include "base/time/time.h"
 #include "ui/gfx/gfx_export.h"
@@ skipping 10 matching lines @@
 
   // Get the time of the most recent screen refresh, along with the time
   // between consecutive refreshes. The callback is called as soon as
   // the data is available: it could be immediately from this method,
   // later via a PostTask to the current MessageLoop, or never (if we have
   // no data source). We provide the strong guarantee that the callback will
   // not be called once the instance of this class is destroyed.
   virtual void GetVSyncParameters(const UpdateVSyncCallback& callback) = 0;
 };
 
+// Provides a contant timebase and interval.
+class GFX_EXPORT FixedVSyncProvider : public VSyncProvider {

[sunnyps, 2016/08/25 01:59:29]

Do we even need FixedVSyncProvider? If a vsync provider is not present the
synthetic BFS will use BeginFrameArgs::DefaultInterval (16667 us) so just
modifying that should be sufficient, no?

[danakj, 2016/08/25 02:04:52]

BeginFrameArgs::DefaultInterval is used all over the place tho, right? I find
changing that for branching to be scary.

[brianderson, 2016/08/25 05:09:50]

I did it this way because I like the decision to use a fixed provider to be
explicit and in the same place where the other vsync providers are being
selected, but Dana makes a good point about branching.

Going to keep this as-is for now.

+ public:
+  FixedVSyncProvider(base::TimeTicks timebase, base::TimeDelta interval)
+    : timebase_(timebase), interval_(interval) {
+  }
+
+  ~FixedVSyncProvider() override {}
+
+  void GetVSyncParameters(const UpdateVSyncCallback& callback) override;
+
+ private:
+  base::TimeTicks timebase_;
+  base::TimeDelta interval_;
+};
+
 }  // namespace gfx
 
 #endif  // UI_GFX_VSYNC_PROVIDER_H_