Limit heap growth when uploading with SetSubData.

Removes unbounded heap growth by capping the heap
size to 4MB and falling back to direct allocation for larger
requests.

BUG=dawn:239

Change-Id: I9ae660809e3c0c539fbcfbee4afcf6fb1f466355
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12720
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

src/tests/perf_tests/BufferUploadPerf.cpp

@@ -142,10 +142,6 @@ void BufferUploadPerf::Step() {
 }
 
 TEST_P(BufferUploadPerf, Run) {
-    // TODO(crbug.com/dawn/239): Investigate why large buffer uploads via SetSubData fail.
-    DAWN_SKIP_TEST_IF(GetParam().uploadMethod == UploadMethod::SetSubData &&
-                      GetParam().uploadSize == UploadSize::BufferSize_16MB);
-
     RunTest();
 }