Make sure GpuMemoryBufferImpl is unmapped on destruction.

BUG=475633

Review URL: https://codereview.chromium.org/1132353003

Cr-Commit-Position: refs/heads/master@{#330782}

content/common/gpu/client/gpu_memory_buffer_impl.cc

@@ -36,6 +36,7 @@ GpuMemoryBufferImpl::GpuMemoryBufferImpl(gfx::GpuMemoryBufferId id,
 }
 
 GpuMemoryBufferImpl::~GpuMemoryBufferImpl() {
+  DCHECK(!mapped_);
   callback_.Run(destruction_sync_point_);
 }