GrBufferAllocPool: use _throw calloc() if unchecked.

Looks like this code is using sk_calloc(), NULL on failure, accidentally
instead of sk_calloc_throw().  We're using sk_malloc_throw() in the parallel
code path, so it really seems like we're not checking the result pointer.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2152753002

Review-Url: https://codereview.chromium.org/2152753002
diff --git a/src/gpu/GrBufferAllocPool.cpp b/src/gpu/GrBufferAllocPool.cpp
index d19604c..bb73a10 100644
--- a/src/gpu/GrBufferAllocPool.cpp
+++ b/src/gpu/GrBufferAllocPool.cpp
@@ -281,7 +281,7 @@
     sk_free(fCpuData);
     if (newSize) {
         if (fGpu->caps()->mustClearUploadedBufferData()) {
-            fCpuData = sk_calloc(newSize);
+            fCpuData = sk_calloc_throw(newSize);
         } else {
             fCpuData = sk_malloc_throw(newSize);
         }