add the sub run allocator to OwnedArenas operator =

This was an oversight in CL:
https://skia-review.googlesource.com/c/skia/+/383757

Change-Id: Icb7b22218a04ef967fda6d998b1f329fdd334970
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383956
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>

src/gpu/GrRecordingContext.cpp

@@ -116,6 +116,7 @@ GrRecordingContext::OwnedArenas::~OwnedArenas() {}
 GrRecordingContext::OwnedArenas& GrRecordingContext::OwnedArenas::operator=(OwnedArenas&& a) {
     fOpMemoryPool = std::move(a.fOpMemoryPool);
     fRecordTimeAllocator = std::move(a.fRecordTimeAllocator);
+    fRecordTimeSubRunAllocator = std::move(a.fRecordTimeSubRunAllocator);
     return *this;
 }