Allow Windows to use system Vsync for a single window each swap

ui/gl/gl_surface_egl.cc

Index: ui/gl/gl_surface_egl.cc
diff --git a/ui/gl/gl_surface_egl.cc b/ui/gl/gl_surface_egl.cc
index 8c08ff4981c19a04101deca89b10581bd259577a..c34ca0e70e7a25396a82cd9310a90c2ef0630cd6 100644
--- a/ui/gl/gl_surface_egl.cc
+++ b/ui/gl/gl_surface_egl.cc
@@ -54,6 +54,8 @@ using ui::GetLastEGLErrorString;
 
 namespace gfx {
 
+unsigned int NativeViewGLSurfaceEGL::current_swap_generation_ = 0;
+
 namespace {
 
 EGLConfig g_config;
@@ -286,7 +288,9 @@ NativeViewGLSurfaceEGL::NativeViewGLSurfaceEGL(EGLNativeWindowType window)
       surface_(NULL),
       supports_post_sub_buffer_(false),
       config_(NULL),
-      size_(1, 1) {
+      size_(1, 1),
+      swap_interval_(1),
+      swap_generation_(0) {
 #if defined(OS_ANDROID)
   if (window)
     ANativeWindow_acquire(window);
@@ -450,12 +454,37 @@ bool NativeViewGLSurfaceEGL::SwapBuffers() {
       "width", GetSize().width(),
       "height", GetSize().height());
 
+#if defined(OS_WIN)
+  bool force_no_vsync = false;
+  if (swap_interval_ != 0) {
+    // This code is a simple way of enforcing that only one surface actually
+    // vsyncs per frame. This provides single window cases a stable refresh
+    // while allowing multi-window cases to not slow down due to multiple syncs
+    // on a single thread. A better way to fix this problem would be to have
+    // each surface present on its own thread.
+    if (current_swap_generation_ == swap_generation_) {
+      current_swap_generation_++;
+    } else {
+      force_no_vsync = true;
+      eglSwapInterval(GetDisplay(), 0);
+    }
+
+    swap_generation_ = current_swap_generation_;
+  }
+#endif
+
   if (!eglSwapBuffers(GetDisplay(), surface_)) {
     DVLOG(1) << "eglSwapBuffers failed with error "
              << GetLastEGLErrorString();
     return false;
   }
 
+#if defined(OS_WIN)
+  if (force_no_vsync) {
+    eglSwapInterval(GetDisplay(), swap_interval_);
+  }
+#endif
+
   return true;
 }
 
@@ -530,6 +559,10 @@ VSyncProvider* NativeViewGLSurfaceEGL::GetVSyncProvider() {
   return vsync_provider_.get();
 }
 
+void NativeViewGLSurfaceEGL::SetSwapInterval(int interval) {
+  swap_interval_ = interval;
+}
+
 NativeViewGLSurfaceEGL::~NativeViewGLSurfaceEGL() {
   Destroy();
 #if defined(OS_ANDROID)