Update from https://crrev.com/306655

cc/resources/raster_source_helper.cc

+// Copyright 2014 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.
+
+#include "cc/resources/raster_source_helper.h"
+
+#include "base/debug/trace_event.h"
+#include "cc/debug/debug_colors.h"
+#include "third_party/skia/include/core/SkCanvas.h"
+#include "ui/gfx/geometry/rect_conversions.h"
+#include "ui/gfx/skia_util.h"
+
+namespace cc {
+
+void RasterSourceHelper::PrepareForPlaybackToCanvas(
+    SkCanvas* canvas,
+    const gfx::Rect& canvas_rect,
+    const gfx::Rect& source_rect,
+    float contents_scale,
+    SkColor background_color,
+    bool clear_canvas_with_debug_color,
+    bool requires_clear) {
+  canvas->discard();
+  if (clear_canvas_with_debug_color) {
+    // Any non-painted areas in the content bounds will be left in this color.
+    canvas->clear(DebugColors::NonPaintedFillColor());
+  }
+
+  // If this raster source has opaque contents, it is guaranteeing that it will
+  // draw an opaque rect the size of the layer.  If it is not, then we must
+  // clear this canvas ourselves.
+  if (requires_clear) {
+    TRACE_EVENT_INSTANT0("cc", "SkCanvas::clear", TRACE_EVENT_SCOPE_THREAD);
+    // Clearing is about ~4x faster than drawing a rect even if the content
+    // isn't covering a majority of the canvas.
+    canvas->clear(SK_ColorTRANSPARENT);
+  } else {
+    // Even if completely covered, for rasterizations that touch the edge of the
+    // layer, we also need to raster the background color underneath the last
+    // texel (since the recording won't cover it) and outside the last texel
+    // (due to linear filtering when using this texture).
+    gfx::Rect content_rect =
+        gfx::ToEnclosingRect(gfx::ScaleRect(source_rect, contents_scale));
+
+    // The final texel of content may only be partially covered by a
+    // rasterization; this rect represents the content rect that is fully
+    // covered by content.
+    gfx::Rect deflated_content_rect = content_rect;
+    deflated_content_rect.Inset(0, 0, 1, 1);
+    if (!deflated_content_rect.Contains(canvas_rect)) {
+      if (clear_canvas_with_debug_color) {
+        // Any non-painted areas outside of the content bounds are left in
+        // this color.  If this is seen then it means that cc neglected to
+        // rerasterize a tile that used to intersect with the content rect
+        // after the content bounds grew.
+        canvas->save();
+        canvas->translate(-canvas_rect.x(), -canvas_rect.y());
+        canvas->clipRect(gfx::RectToSkRect(content_rect),
+                         SkRegion::kDifference_Op);
+        canvas->drawColor(DebugColors::MissingResizeInvalidations(),
+                          SkXfermode::kSrc_Mode);
+        canvas->restore();
+      }
+
+      // Drawing at most 2 x 2 x (canvas width + canvas height) texels is 2-3X
+      // faster than clearing, so special case this.
+      canvas->save();
+      canvas->translate(-canvas_rect.x(), -canvas_rect.y());
+      gfx::Rect inflated_content_rect = content_rect;
+      inflated_content_rect.Inset(0, 0, -1, -1);
+      canvas->clipRect(gfx::RectToSkRect(inflated_content_rect),
+                       SkRegion::kReplace_Op);
+      canvas->clipRect(gfx::RectToSkRect(deflated_content_rect),
+                       SkRegion::kDifference_Op);
+      canvas->drawColor(background_color, SkXfermode::kSrc_Mode);
+      canvas->restore();
+    }
+  }
+}
+
+}  // namespace cc