media: Add UnsafeSharedMemoryRegion BitstreamBuffer ctor

Add an UnsafeSharedMemoryRegion constructor for BitstreamBuffer.
While BitstreamBuffer continues to represent its shared memory
internally by base::subtle::PlatformSharedMemoryRegion due to
being used sometimes read-only and sometimes writable, this
constructor makes the bitstream buffer easier to use in the common
case of client/tests which create a buffer backed by unsafe
(writable) shared memory.

Bug: 849207
Change-Id: I271bcd389e936ab2e7207bb2684fc6bcd8c2d529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695264
Commit-Queue: Matthew Cary (CET) <mattcary@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681431}

media/base/bitstream_buffer.cc

@@ -23,6 +23,18 @@ BitstreamBuffer::BitstreamBuffer(
       offset_(offset),
       presentation_timestamp_(presentation_timestamp) {}
 
+BitstreamBuffer::BitstreamBuffer(int32_t id,
+                                 base::UnsafeSharedMemoryRegion region,
+                                 size_t size,
+                                 off_t offset,
+                                 base::TimeDelta presentation_timestamp)
+    : id_(id),
+      region_(base::UnsafeSharedMemoryRegion::TakeHandleForSerialization(
+          std::move(region))),
+      size_(size),
+      offset_(offset),
+      presentation_timestamp_(presentation_timestamp) {}
+
 BitstreamBuffer::BitstreamBuffer(int32_t id,
                                  base::SharedMemoryHandle handle,
                                  bool read_only,

media/base/bitstream_buffer.h

@@ -11,6 +11,7 @@
 #include "base/macros.h"
 #include "base/memory/platform_shared_memory_region.h"
 #include "base/memory/scoped_refptr.h"
+#include "base/memory/unsafe_shared_memory_region.h"
 #include "base/time/time.h"
 #include "media/base/decoder_buffer.h"
 #include "media/base/decrypt_config.h"
@@ -42,6 +43,13 @@ class MEDIA_EXPORT BitstreamBuffer {
                   off_t offset = 0,
                   base::TimeDelta presentation_timestamp = kNoTimestamp);
 
+  // As above, creating by unwrapping a base::UnsafeSharedMemoryRegion.
+  BitstreamBuffer(int32_t id,
+                  base::UnsafeSharedMemoryRegion region,
+                  size_t size,
+                  off_t offset = 0,
+                  base::TimeDelta presentation_timestamp = kNoTimestamp);
+
   // As above, but creates by duplicating a SharedMemoryHandle.
   // TODO(https://crbug.com/793446): remove once legacy shared memory has been
   // converted.