[Paint Preview] Omit Alpha channel

Frames can't have transparent backgrounds so omit the alpha channel to
simplify things.

Change-Id: Ic9d1de433144123d5572a9557c03598fc82de635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463601Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817092}

components/services/paint_preview_compositor/paint_preview_compositor_impl.cc

@@ -86,8 +86,10 @@ base::Optional<SkBitmap> CreateBitmap(sk_sp<SkPicture> skp,
                                       float scale_factor) {
   TRACE_EVENT0("paint_preview", "PaintPreviewCompositorImpl::CreateBitmap");
   SkBitmap bitmap;
-  if (!bitmap.tryAllocPixels(
-          SkImageInfo::MakeN32Premul(clip_rect.width(), clip_rect.height()))) {
+  // Use N32 rather than an alpha color type as frames cannot have transparent
+  // backgrounds.
+  if (!bitmap.tryAllocPixels(SkImageInfo::MakeN32(
+          clip_rect.width(), clip_rect.height(), kOpaque_SkAlphaType))) {
     return base::nullopt;
   }
   SkCanvas canvas(bitmap);

components/services/paint_preview_compositor/paint_preview_compositor_impl_unittest.cc

@@ -599,7 +599,8 @@ TEST(PaintPreviewCompositorTest, TestComposite) {
   gfx::Rect rect = gfx::ScaleToEnclosingRect(
       gfx::Rect(root_frame_scroll_extent), scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   DrawDummyTestPicture(&canvas, SK_ColorDKGRAY, root_frame_scroll_extent);
@@ -670,7 +671,8 @@ TEST(PaintPreviewCompositorTest, TestCompositeWithMemoryBuffer) {
   gfx::Rect rect = gfx::ScaleToEnclosingRect(
       gfx::Rect(root_frame_scroll_extent), scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   DrawDummyTestPicture(&canvas, SK_ColorDKGRAY, root_frame_scroll_extent);
@@ -719,7 +721,8 @@ TEST(PaintPreviewCompositorTest, TestCompositeMainFrameNoDependencies) {
   gfx::Rect rect = gfx::ScaleToEnclosingRect(
       gfx::Rect(root_frame_scroll_extent), scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   DrawDummyTestPicture(&canvas, SK_ColorDKGRAY, root_frame_scroll_extent);
@@ -773,7 +776,8 @@ TEST(PaintPreviewCompositorTest, TestCompositeMainFrameOneDependency) {
   gfx::Rect rect = gfx::ScaleToEnclosingRect(
       gfx::Rect(root_frame_scroll_extent), scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   DrawDummyTestPicture(&canvas, SK_ColorDKGRAY, root_frame_scroll_extent);
@@ -831,7 +835,8 @@ TEST(PaintPreviewCompositorTest, TestCompositeMainFrameOneDependencyScrolled) {
   gfx::Rect rect = gfx::ScaleToEnclosingRect(
       gfx::Rect(root_frame_scroll_extent), scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   DrawDummyTestPicture(&canvas, SK_ColorDKGRAY, root_frame_scroll_extent);
@@ -892,7 +897,8 @@ TEST(PaintPreviewCompositorTest,
   gfx::Rect rect =
       gfx::ScaleToEnclosingRect(root_frame_clip_rect, scale_factor);
   SkBitmap bitmap;
-  bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+  bitmap.allocPixels(
+      SkImageInfo::MakeN32(rect.width(), rect.height(), kOpaque_SkAlphaType));
   SkCanvas canvas(bitmap);
   canvas.scale(scale_factor, scale_factor);
   // Offset the canvas to simulate the root frame being scrolled.