Do not consolidate data in BMPImageDecoder

Source/platform/image-decoders/FastSharedBufferReader.h

Index: Source/platform/image-decoders/FastSharedBufferReader.h
diff --git a/Source/platform/image-decoders/FastSharedBufferReader.h b/Source/platform/image-decoders/FastSharedBufferReader.h
index 254476b435615daeabbfc07fce8ded953dd8729d..9cab891679de5d20688c52e5b1bb81cbb5050888 100644
--- a/Source/platform/image-decoders/FastSharedBufferReader.h
+++ b/Source/platform/image-decoders/FastSharedBufferReader.h
@@ -46,6 +46,8 @@ class PLATFORM_EXPORT FastSharedBufferReader {
 public:
     FastSharedBufferReader(PassRefPtr<SharedBuffer> data);
 
+    void setData(SharedBuffer*);

[Peter Kasting, 2015/09/02 22:08:54]

Is it really correct that the constructor takes a PassRefPtr but this takes a
raw pointer?  I don't know Blink's pointer type semantics but that looks
strange.

[scroggo_chromium, 2015/09/02 23:01:07]

Me neither...
Agreed. ImageDecoder::setData also takes a raw pointer, and this method is
called by that one. We could wrap that pointer...

[Peter Kasting, 2015/09/02 23:48:44]

Definitely at least figure out precisely what the code is doing today and why. 
That way we'll know whether this difference is important.

[scroggo_chromium, 2015/09/03 22:04:19]

I just talked this over with a couple of people, and here is my understanding:

m_data is a RefPtr<SharedBuffer>. As is written in this patch set, setData takes
a raw pointer. When I do the assignment to m_data, that increases the ref count
so the FastSharedBufferReader has a reference.

If I used a PassRefPtr<SharedBuffer> instead, passing the raw pointer (from
BMPImageReader::setData) to FastSharedBufferReader::setData implicitly creates a
PassRefPtr, which increases the ref count. When that gets assigned to m_data,
that does *not* increase the ref count, but it does tell the PassRefPtr not to
decrement it (which it would do otherwise - this is important, because it means
if data == m_data, so we do not do the assignment, the PassRefPtr *does*
decrement it).

So, both do essentially the same thing. The PassRefPtr approach increments the
pointer earlier (and therefore adds some unnecessary churn, in the case where it
is the same as m_data). That said, I think consistency is better, and I
understand we generally do not want to use raw pointers. The latest patch set
makes this parameter a PassRefPtr.

+
     // Returns a consecutive buffer that carries the data starting
     // at |dataPosition| with |length| bytes.
     // This method returns a pointer to a memory segment stored in