Motown: Remove LPCM optimizations, fix prepare, add flush, add ActiveMultistreamSink model/stage
- Removed LPCM optimizations
- Converted LpcmReformatter from LPCM transport to Packet transport
- Added ActiveMultistreamSink model and hosting stage for system mixer use (needs debugging)
- Moved graph building from Engine to new Graph class
- Established new graph-building threading model (not thread-safe)
- Fixed Prepare logic
- Added Unprepare
- Disallowing disconnection of prepared inputs/outputs
- Added Flush
- Now using InputRef and OutputRef internally
- Renamed:
    Engine::Part -> PartRef
    Engine::Input -> InputRef
    Engine::Output -> OutputRef
    StageInput -> Input
    StageOutput -> Output
    Allocator -> PayloadAllocator
- Many instances of uint32_t and uint64_t replaced with size_t

R=johngro@google.com

Review URL: https://codereview.chromium.org/1678433002 .
diff --git a/services/media/framework/packet.cc b/services/media/framework/packet.cc
index 9b02493..32204bc 100644
--- a/services/media/framework/packet.cc
+++ b/services/media/framework/packet.cc
@@ -3,8 +3,8 @@
 // found in the LICENSE file.
 
 #include "base/logging.h"
-#include "services/media/framework/allocator.h"
 #include "services/media/framework/packet.h"
+#include "services/media/framework/payload_allocator.h"
 
 namespace mojo {
 namespace media {
@@ -15,9 +15,9 @@
       int64_t presentation_time,
       uint64_t duration,
       bool end_of_stream,
-      uint64_t size,
+      size_t size,
       void* payload,
-      Allocator* allocator) :
+      PayloadAllocator* allocator) :
       presentation_time_(presentation_time),
       duration_(duration),
       end_of_stream_(end_of_stream),
@@ -35,7 +35,7 @@
 
   bool end_of_stream() const override { return end_of_stream_; }
 
-  uint64_t size() const override { return size_; }
+  size_t size() const override { return size_; }
 
   void* payload() const override { return payload_; }
 
@@ -52,9 +52,9 @@
   int64_t presentation_time_;
   uint64_t duration_;
   bool end_of_stream_;
-  uint64_t size_;
+  size_t size_;
   void* payload_;
-  Allocator* allocator_;
+  PayloadAllocator* allocator_;
 };
 
 // static
@@ -62,9 +62,9 @@
     int64_t presentation_time,
     uint64_t duration,
     bool end_of_stream,
-    uint64_t size,
+    size_t size,
     void* payload,
-    Allocator* allocator) {
+    PayloadAllocator* allocator) {
   DCHECK(payload == nullptr || allocator != nullptr);
   return PacketPtr(new PacketImpl(
       presentation_time,
@@ -80,7 +80,7 @@
     int64_t presentation_time,
     uint64_t duration,
     bool end_of_stream,
-    uint64_t size,
+    size_t size,
     void* payload) {
   return PacketPtr(new PacketImpl(
       presentation_time,