Use a layer transform for the waiting throbber

ui/gfx/paint_throbber.cc

Index: ui/gfx/paint_throbber.cc
diff --git a/ui/gfx/paint_throbber.cc b/ui/gfx/paint_throbber.cc
index 0c4505af6e48352bbe3fd1b880ce0d9a3e9b4e5c..b75d52d32ec86112cec066f951bb6a824e464824 100644
--- a/ui/gfx/paint_throbber.cc
+++ b/ui/gfx/paint_throbber.cc
@@ -26,35 +26,6 @@ const int64_t kArcTimeMs = 666;
 // rotation.
 const int64_t kRotationTimeMs = 1568;
 
-void PaintArc(Canvas* canvas,
-              const Rect& bounds,
-              SkColor color,
-              SkScalar start_angle,
-              SkScalar sweep) {
-  // Stroke width depends on size.
-  // . For size < 28:          3 - (28 - size) / 16
-  // . For 28 <= size:         (8 + size) / 12
-  SkScalar stroke_width = bounds.width() < 28
-                              ? 3.0 - SkIntToScalar(28 - bounds.width()) / 16.0
-                              : SkIntToScalar(bounds.width() + 8) / 12.0;
-  Rect oval = bounds;
-  // Inset by half the stroke width to make sure the whole arc is inside
-  // the visible rect.
-  int inset = SkScalarCeilToInt(stroke_width / 2.0);
-  oval.Inset(inset, inset);
-
-  SkPath path;
-  path.arcTo(RectToSkRect(oval), start_angle, sweep, true);
-
-  SkPaint paint;
-  paint.setColor(color);
-  paint.setStrokeCap(SkPaint::kRound_Cap);
-  paint.setStrokeWidth(stroke_width);
-  paint.setStyle(SkPaint::kStroke_Style);
-  paint.setAntiAlias(true);
-  canvas->DrawPath(path, paint);
-}
-
 void CalculateWaitingAngles(const base::TimeDelta& elapsed_time,
                             int64_t* start_angle,
                             int64_t* sweep) {
@@ -110,12 +81,41 @@ void PaintThrobberSpinningWithStartAngle(Canvas* canvas,
   // To keep the sweep smooth, we have an additional rotation after each
   // |arc_time| period has elapsed. See SVG's 'rot' animation.
   int64_t rot_keyframe = (elapsed_time / (arc_time * 2)) % 4;
-  PaintArc(canvas, bounds, color, start_angle + rot_keyframe * kMaxArcSize,
-           sweep);
+  PaintThrobberArc(canvas, bounds, color,
+                   start_angle + rot_keyframe * kMaxArcSize, sweep);
 }
 
 }  // namespace
 
+void PaintThrobberArc(Canvas* canvas,
+                      const Rect& bounds,
+                      SkColor color,
+                      SkScalar start_angle,
+                      SkScalar sweep) {
+  // Stroke width depends on size.
+  // . For size < 28:          3 - (28 - size) / 16
+  // . For 28 <= size:         (8 + size) / 12
+  SkScalar stroke_width = bounds.width() < 28
+                              ? 3.0 - SkIntToScalar(28 - bounds.width()) / 16.0
+                              : SkIntToScalar(bounds.width() + 8) / 12.0;
+  Rect oval = bounds;
+  // Inset by half the stroke width to make sure the whole arc is inside
+  // the visible rect.
+  int inset = SkScalarCeilToInt(stroke_width / 2.0);
+  oval.Inset(inset, inset);
+
+  SkPath path;
+  path.arcTo(RectToSkRect(oval), start_angle, sweep, true);
+
+  SkPaint paint;
+  paint.setColor(color);
+  paint.setStrokeCap(SkPaint::kRound_Cap);
+  paint.setStrokeWidth(stroke_width);
+  paint.setStyle(SkPaint::kStroke_Style);
+  paint.setAntiAlias(true);
+  canvas->DrawPath(path, paint);
+}
+
 void PaintThrobberSpinning(Canvas* canvas,
                            const Rect& bounds,
                            SkColor color,
@@ -131,7 +131,7 @@ void PaintThrobberWaiting(Canvas* canvas,
     const Rect& bounds, SkColor color, const base::TimeDelta& elapsed_time) {
   int64_t start_angle = 0, sweep = 0;
   CalculateWaitingAngles(elapsed_time, &start_angle, &sweep);
-  PaintArc(canvas, bounds, color, start_angle, sweep);
+  PaintThrobberArc(canvas, bounds, color, start_angle, sweep);
 }
 
 void PaintThrobberSpinningAfterWaiting(Canvas* canvas,