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/tests/unittests/RingBufferTests.cpp

@@ -33,7 +33,7 @@ namespace {
 class RingBufferTests : public testing::Test {
   protected:
     void SetUp() override {
-        // TODO(b-brber): Create this device through the adapter.
+        // TODO(bryan.bernhart@intel.com): Create this device through the adapter.
         mDevice = std::make_unique<null::Device>(/*adapter*/ nullptr);
     }
 
@@ -165,7 +165,7 @@ TEST_F(RingBufferTests, RingBufferSubAlloc) {
 
     // In this example, Tick(8) could not reclaim the wasted bytes. The wasted bytes
     // were add to F9's sub-allocation.
-    // TODO(b-brber): Decide if Tick(8) should free these wasted bytes.
+    // TODO(bryan.bernhart@intel.com): Decide if Tick(8) should free these wasted bytes.
 
     ASSERT_EQ(offset, 0u);
     ASSERT_EQ(buffer->GetUsedSize(), frameSizeInBytes * maxNumOfFrames);