Memory manager: buffer uploads (Metal+refactor) - Part 3

Manages a single persistently mapped GPU heap which is sub-allocated
inside of ring-buffer for uploads. To handle larger buffers without additional
unused heaps, ring buffers are created on-demand.

BUG=dawn:28
TEST=dawn_unittests

Change-Id: If4d3e717186895b1409502c1dea5ab751a4776b2
Reviewed-on: https://dawn-review.googlesource.com/c/4440
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

src/dawn_native/opengl/DeviceGL.cpp

@@ -44,6 +44,9 @@ namespace dawn_native { namespace opengl {
         // on a serial that doesn't have a corresponding fence enqueued. Force all
         // operations to look as if they were completed (because they were).
         mCompletedSerial = mLastSubmittedSerial + 1;
+
+        mDynamicUploader = nullptr;
+
         Tick();
     }