SkBitmap move

Running `Release/dm --gpu 0`, the number of times we call
	SkBitmap::operator=(const SkBitmap&)
(which refs the pixelref) is reduced from ~214929 to ~214626.

Review URL: https://codereview.chromium.org/1514503004

include/core/SkBitmap.h

@@ -54,12 +54,24 @@ public:
      */
     SkBitmap(const SkBitmap& src);
 
+    /**
+     *  Copy the settings from the src into this bitmap. If the src has pixels
+     *  allocated, ownership of the pixels will be taken.
+     */
+    SkBitmap(SkBitmap&& src);
+
     ~SkBitmap();
 
-    /** Copies the src bitmap into this bitmap. Ownership of the src bitmap's pixels remains
-        with the src bitmap.
+    /** Copies the src bitmap into this bitmap. Ownership of the src
+        bitmap's pixels is shared with the src bitmap.
     */
     SkBitmap& operator=(const SkBitmap& src);
+
+    /** Copies the src bitmap into this bitmap. Takes ownership of the src
+        bitmap's pixels.
+    */
+    SkBitmap& operator=(SkBitmap&& src);
+
     /** Swap the fields of the two bitmaps. This routine is guaranteed to never fail or throw.
     */
     //  This method is not exported to java.

src/core/SkBitmap.cpp

@@ -39,6 +39,8 @@ SkBitmap::SkBitmap(const SkBitmap& src) {
     SkDEBUGCODE(this->validate();)
 }
 
+SkBitmap::SkBitmap(SkBitmap&& other) : SkBitmap() { this->swap(other); }
+
 SkBitmap::~SkBitmap() {
     SkDEBUGCODE(this->validate();)
     this->freePixels();
@@ -72,6 +74,14 @@ SkBitmap& SkBitmap::operator=(const SkBitmap& src) {
     return *this;
 }
 
+SkBitmap& SkBitmap::operator=(SkBitmap&& other) {
+    if (this != &other) {
+        this->swap(other);
+        other.reset();
+    }
+    return *this;
+}
+
 void SkBitmap::swap(SkBitmap& other) {
     SkTSwap(fColorTable, other.fColorTable);
     SkTSwap(fPixelRef, other.fPixelRef);