SkDiscardablePixelRef objects are now marked immutable.

I also specifically mentioned in the SkImageGenerator documentation
that it should always return the same information on subsequent calls
to getInfo() and getPixels().

Motivation: When a bitmap is recorded into a SkPicture, it will choose
whether to shallow- or deep-copy itself into the SkBitmapHeap based on
the immutable flag.  Failing to set it would make picture recording slow.

BUG=
R=fmalita@google.com, reed@google.com, scroggo@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/99303003

git-svn-id: http://skia.googlecode.com/svn/trunk@12445 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkImageGenerator.h

@@ -37,6 +37,9 @@ public:
      *  Return some information about the image, allowing the owner of
      *  this object to allocate pixels.
      *
+     *  Repeated calls to this function should give the same results,
+     *  allowing the PixelRef to be immutable.
+     *
      *  @return false if anything goes wrong.
      */
     virtual bool getInfo(SkImageInfo* info) = 0;
@@ -46,6 +49,9 @@ public:
      *  least (info.fHeight - 1) * rowBytes + (info.fWidth *
      *  bytesPerPixel)
      *
+     *  Repeated calls to this function should give the same results,
+     *  allowing the PixelRef to be immutable.
+     *
      *  @param info A description of the format (config, size)
      *         expected by the caller.  This can simply be identical
      *         to the info returned by getInfo().

src/lazy/SkDiscardablePixelRef.cpp

@@ -20,6 +20,9 @@ SkDiscardablePixelRef::SkDiscardablePixelRef(SkImageGenerator* generator,
     SkASSERT(fGenerator != NULL);
     SkASSERT(fSize > 0);
     SkASSERT(fRowBytes > 0);
+    // The SkImageGenerator contract requires fGenerator to always
+    // decode the same image on each call to getPixels().
+    this->setImmutable();
 }
 
 SkDiscardablePixelRef::~SkDiscardablePixelRef() {