Add offset support to BitstreamBuffer.

content/common/gpu/media/gpu_video_decode_accelerator.cc

Index: content/common/gpu/media/gpu_video_decode_accelerator.cc
diff --git a/content/common/gpu/media/gpu_video_decode_accelerator.cc b/content/common/gpu/media/gpu_video_decode_accelerator.cc
index 5424a5ff32d801d846662f7fb95b910caf19e7ef..65a3f54228bedc4674e65b091396794ceaeeaa1e 100644
--- a/content/common/gpu/media/gpu_video_decode_accelerator.cc
+++ b/content/common/gpu/media/gpu_video_decode_accelerator.cc
@@ -509,9 +509,9 @@ void GpuVideoDecodeAccelerator::OnDecode(
     return;
   }
 
-  media::BitstreamBuffer bitstream_buffer(params.bitstream_buffer_id,
-                                          params.buffer_handle, params.size,
-                                          params.presentation_timestamp);
+  media::BitstreamBuffer bitstream_buffer(
+      params.bitstream_buffer_id, params.buffer_handle, params.size,
+      checked_cast<off_t>(params.offset), params.presentation_timestamp);

[dcheng, 2016/01/25 19:09:18]

Should this really crash the GPU (I think?) process? It seems like it should
just be returning and failing gracefully.

I think it'd be better to have a ParamTraits for BitstreamBuffer, so the
validation code can be in a common logic, rather than interspersed through
various IPC message handlers.

[Owen Lin, 2016/01/29 05:38:17]

Change to returning error.

Besides, I also tried the ParamTrais
(https://codereview.chromium.org/1645873002/) 

However, the validation rules for a bitstream buffer are:

1. the handle is valid
2. the id is positive.
3. the offset is in the range of off_t.

However a BitstreamBuffer may not satisfy 1 and 2. That's means we may Write() a
BitstreamBuffer but fail to Read() it back. This sounds weired. So, I still
prefer leaving the validation job in gpu_video|jpeg_decode_accelerator.cc, or
even defer it to the real accelerators.

How do you think?

   if (!params.key_id.empty()) {
     bitstream_buffer.SetDecryptConfig(
         media::DecryptConfig(params.key_id, params.iv, params.subsamples));