Take advantage of MojoGetBufferInformation

mojo/services/media/common/cpp/mapped_shared_buffer.cc

Index: mojo/services/media/common/cpp/mapped_shared_buffer.cc
diff --git a/mojo/services/media/common/cpp/mapped_shared_buffer.cc b/mojo/services/media/common/cpp/mapped_shared_buffer.cc
index 5d0cb4b6f4ea1dac946a7f74c477c58b522f902c..976858ac6b7ba873ed519ffacd1a3e7749c4ae20 100644
--- a/mojo/services/media/common/cpp/mapped_shared_buffer.cc
+++ b/mojo/services/media/common/cpp/mapped_shared_buffer.cc
@@ -18,25 +18,30 @@ void MappedSharedBuffer::InitNew(uint64_t size) {
   buffer_.reset(new SharedBuffer(size));
   handle_.reset();
 
-  InitInternal(buffer_->handle, size);
+  InitInternal(buffer_->handle);
 }
 
-void MappedSharedBuffer::InitFromHandle(
-    ScopedSharedBufferHandle handle,
-    uint64_t size) {
+void MappedSharedBuffer::InitFromHandle(ScopedSharedBufferHandle handle) {
   MOJO_DCHECK(handle.is_valid());
-  MOJO_DCHECK(size > 0);
 
   buffer_.reset();
   handle_ = handle.Pass();
 
-  InitInternal(handle_, size);
+  InitInternal(handle_);
 }
 
-void MappedSharedBuffer::InitInternal(
-    ScopedSharedBufferHandle& handle,
-    uint64_t size) {
+void MappedSharedBuffer::InitInternal(const ScopedSharedBufferHandle& handle) {
   MOJO_DCHECK(handle.is_valid());
+
+  // Query the buffer for its size.
+  // TODO(johngro) :  It would be nice if we could do something other than
+  // DCHECK if things don't go exactly our way.
+  MojoBufferInformation info;
+  MojoResult res = MojoGetBufferInformation(handle.get().value(),
+                                            &info,
+                                            sizeof(info));
+  uint64_t size = info.num_bytes;
+  MOJO_DCHECK(res == MOJO_RESULT_OK);
   MOJO_DCHECK(size > 0);
 
   size_ = size;
@@ -45,7 +50,7 @@ void MappedSharedBuffer::InitInternal(
   void* ptr;
   auto result = MapBuffer(
       handle.get(),
-      0, // offset
+      0,  // offset
       size,
       &ptr,
       MOJO_MAP_BUFFER_FLAG_NONE);
@@ -100,5 +105,5 @@ uint64_t MappedSharedBuffer::OffsetFromPtr(void *ptr) const {
 
 void MappedSharedBuffer::OnInit() {}
 
-} // namespace media
-} // namespace mojo
+}  // namespace media
+}  // namespace mojo