Bug 1197125: [webm]. Don't load the entire webm in RAM. r=kinetik

Instead we parse it in chunks of 1MiB max.

dom/media/webm/WebMBufferedParser.cpp

@@ -415,9 +415,16 @@ void WebMBufferedState::UpdateIndex(const nsTArray<MediaByteRange>& aRanges, Med
         }
       }
     }
-    nsRefPtr<MediaByteBuffer> bytes = aResource->MediaReadAt(offset, length);
+    while (length > 0) {
-    if(bytes) {
+      static const uint32_t BLOCK_SIZE = 1048576;
+      uint32_t block = std::min(length, BLOCK_SIZE);
+      nsRefPtr<MediaByteBuffer> bytes = aResource->MediaReadAt(offset, block);
+      if (!bytes) {
+        break;
+      }
       NotifyDataArrived(bytes->Elements(), bytes->Length(), offset);
+      length -= bytes->Length();
+      offset += bytes->Length();
     }
   }
 }