gfx: Make ImageSkipRep bitmaps immutable.

This avoids having to copy them when inserting them into a recording
during paint.

This patch changes the time to do UpdateLayers for a single tab
loading spinner from 0.206279ms to 0.163527 on a linux z620, giving
a 21% reduction in frame record time. I used a mean of 2000 samples
(throwing out the first 100) in each run to reduce noise.

R=sky
BUG=466426

Review URL: https://codereview.chromium.org/1128403003

Cr-Commit-Position: refs/heads/master@{#328887}

4 files changed, 7 insertions, 8 deletions

diff --git a/ui/gfx/image/image_skia.cc b/ui/gfx/image/image_skia.cc
index 5e4410a..7158f24 100644
--- a/ui/gfx/image/image_skia.cc
+++ b/ui/gfx/image/image_skia.cc
@@ -466,11 +466,11 @@ void ImageSkia::Init(const ImageSkiaRep& image_rep) {
   storage_->image_reps().push_back(image_rep);
 }
 
-SkBitmap& ImageSkia::GetBitmap() const {
+const SkBitmap& ImageSkia::GetBitmap() const {
   if (isNull()) {
     // Callers expect a ImageSkiaRep even if it is |isNull()|.
     // TODO(pkotwicz): Fix this.
-    return NullImageRep().mutable_sk_bitmap();
+    return NullImageRep().sk_bitmap();
   }
 
   // TODO(oshima): This made a few tests flaky on Windows.
@@ -481,8 +481,8 @@ SkBitmap& ImageSkia::GetBitmap() const {
 
   ImageSkiaReps::iterator it = storage_->FindRepresentation(1.0f, true);
   if (it != storage_->image_reps().end())
-    return it->mutable_sk_bitmap();
-  return NullImageRep().mutable_sk_bitmap();
+    return it->sk_bitmap();
+  return NullImageRep().sk_bitmap();
 }
 
 bool ImageSkia::CanRead() const {
diff --git a/ui/gfx/image/image_skia.h b/ui/gfx/image/image_skia.h
index fa5d070..941ebc9f 100644
--- a/ui/gfx/image/image_skia.h
+++ b/ui/gfx/image/image_skia.h
@@ -156,7 +156,7 @@ class GFX_EXPORT ImageSkia {
   // If the image rep's bitmap is empty, ImageStorage is set to NULL.
   void Init(const gfx::ImageSkiaRep& image_rep);
 
-  SkBitmap& GetBitmap() const;
+  const SkBitmap& GetBitmap() const;
 
   // Checks if the current thread can read/modify the ImageSkia.
   bool CanRead() const;
diff --git a/ui/gfx/image/image_skia_rep.cc b/ui/gfx/image/image_skia_rep.cc
index 8bc5214..a3f9b472 100644
--- a/ui/gfx/image/image_skia_rep.cc
+++ b/ui/gfx/image/image_skia_rep.cc
@@ -18,11 +18,13 @@ ImageSkiaRep::ImageSkiaRep(const gfx::Size& size, float scale) : scale_(scale) {
   bitmap_.allocN32Pixels(static_cast<int>(size.width() * this->scale()),
                          static_cast<int>(size.height() * this->scale()));
   bitmap_.eraseColor(SK_ColorRED);
+  bitmap_.setImmutable();
 }
 
 ImageSkiaRep::ImageSkiaRep(const SkBitmap& src, float scale)
     : bitmap_(src),
       scale_(scale) {
+  bitmap_.setImmutable();
 }
 
 int ImageSkiaRep::GetWidth() const {
diff --git a/ui/gfx/image/image_skia_rep.h b/ui/gfx/image/image_skia_rep.h
index acf9919..817ddb5 100644
--- a/ui/gfx/image/image_skia_rep.h
+++ b/ui/gfx/image/image_skia_rep.h
@@ -58,9 +58,6 @@ class GFX_EXPORT ImageSkiaRep {
   const SkBitmap& sk_bitmap() const { return bitmap_; }
 
  private:
-  friend class ImageSkia;
-  SkBitmap& mutable_sk_bitmap() { return bitmap_; }
-
   SkBitmap bitmap_;
 
   float scale_;