Add offset support to BitstreamBuffer.

media/base/bitstream_buffer.h

Index: media/base/bitstream_buffer.h
diff --git a/media/base/bitstream_buffer.h b/media/base/bitstream_buffer.h
index fe3c8da358ab021da1255517688e3693a96b4b1e..affe430b96803826985d32b1be1a07a4db692397 100644
--- a/media/base/bitstream_buffer.h
+++ b/media/base/bitstream_buffer.h
@@ -23,19 +23,39 @@ class MEDIA_EXPORT BitstreamBuffer {
  public:
   BitstreamBuffer(int32_t id, base::SharedMemoryHandle handle, size_t size);
 
+  // Constructs a new BitstreamBuffer. The content of the bitstream is located
+  // at |offset| bytes away from the start of the shared memory and the payload
+  // is |size| bytes.
+  BitstreamBuffer(int32_t id,
+                  base::SharedMemoryHandle handle,
+                  size_t size,
+                  off_t offset);

[xhwang, 2016/01/13 18:54:54]

What's the default |presentation_timestamp| in this case? I wonder whether the
convenience of multiple ctor's is worth the confusion...

[xhwang, 2016/01/13 18:54:54]

I never used off_t before. After a bit searching around, it seems off_t is not
in standard C and in posix its' for file sizes. Could you please provide more
details about off_t and why it's the best choice here?

[Owen Lin, 2016/01/14 03:56:34]

When the |presentation_timestamp| is not provided it will be
media::kNoTimestamp().

About the multiple ctros, I believe it because there are default values for
|offset| (as 0) and |presentation_timestamp| (as kNoTimestamp). I originally
thought default argument is not allowed for chromium code style. But I checked
it again, and found it is only banned for virtual functions. 

https://google.github.io/styleguide/cppguide.html#Default_Arguments

I am going to merge those four ctors into just one with default arguments.

However, there is one case cannot be covered, i.e., with timestamp but without
offset. There are only two places of such case in the code base. So I modified
the related codes so that only one ctor is required.

[Owen Lin, 2016/01/14 03:56:34]

This parameter is used to calculate the offset when we call
SharedMemory.MapAt(off_t, size_t);

https://code.google.com/p/chromium/codesearch#chromium/src/base/memory/shared...

It is natural to use the same type of the argument "offset" in MapAt().

+
   BitstreamBuffer(int32_t id,
                   base::SharedMemoryHandle handle,
                   size_t size,
                   base::TimeDelta presentation_timestamp);
 
+  BitstreamBuffer(int32_t id,
+                  base::SharedMemoryHandle handle,
+                  size_t size,
+                  off_t offset,
+                  base::TimeDelta presentation_timestamp);
+
   ~BitstreamBuffer();
 
   void SetDecryptConfig(const DecryptConfig& decrypt_config);
 
   int32_t id() const { return id_; }
   base::SharedMemoryHandle handle() const { return handle_; }
+
+  // The number of bytes of the actual bitstream data. It is the size of the
+  // content instead of the whole shared memory.
   size_t size() const { return size_; }
 
+  // The offset to the start of actual bitstream data in the shared memory.
+  off_t offset() const { return offset_; }
+
   // The timestamp is only valid if it's not equal to |media::kNoTimestamp()|.
   base::TimeDelta presentation_timestamp() const {
     return presentation_timestamp_;
@@ -51,6 +71,7 @@ class MEDIA_EXPORT BitstreamBuffer {
   int32_t id_;
   base::SharedMemoryHandle handle_;
   size_t size_;
+  off_t offset_;
 
   // This is only set when necessary. For example, AndroidVideoDecodeAccelerator
   // needs the timestamp because the underlying decoder may require it to