Fix handle validity check in ChildHistogramFetcherImpl

Invalid Mojo shared memory handles can be "successfully" unwrapped to
invalid base shared memory handles. This may be a little weird, but
that API is going away in favor of unwrapping new base shared memory
types anyway.

So rather than changing the API behavior, this fixes the check in
ChildHistogramFetcherImpl so it doesn't attempt to use an invalid
base::SharedMemoryHandle.

Bug: 835837
Change-Id: If6f4fd5086547e671e4d86363ede9513adb2c054
Reviewed-on: https://chromium-review.googlesource.com/1024114
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552763}

content/child/child_histogram_fetcher_impl.cc

@@ -37,7 +37,7 @@ void ChildHistogramFetcherFactoryImpl::CreateFetcher(
   const MojoResult result = mojo::UnwrapSharedMemoryHandle(
       std::move(buffer_handle), &memory_handle, &memory_size, nullptr);
 
-  if (result == MOJO_RESULT_OK) {
+  if (result == MOJO_RESULT_OK && memory_handle.IsValid()) {
     // This message must be received only once. Multiple calls to create a
     // global allocator will cause a CHECK() failure.
     base::GlobalHistogramAllocator::CreateWithSharedMemoryHandle(memory_handle,