Initialize all members of GpuMemoryBufferHandle in the default ctor

GpuMemoryBufferHandle::offset and GpuMemoryBufferHandle::stride are not
initialized in the default constructor. These uninitialized members may be
serialized later by mojo that makes MSAN bot unhappy.

Bug: 890930
Change-Id: I0663de62997d673b97907a8bd470e9834bdaf638
Reviewed-on: https://chromium-review.googlesource.com/1256786Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595850}

ui/gfx/gpu_memory_buffer.cc

@@ -8,7 +8,8 @@
 
 namespace gfx {
 
-GpuMemoryBufferHandle::GpuMemoryBufferHandle() : type(EMPTY_BUFFER), id(0) {}
+GpuMemoryBufferHandle::GpuMemoryBufferHandle()
+    : type(EMPTY_BUFFER), id(0), offset(0), stride(0) {}
 
 // TODO(crbug.com/863011): Reset |type| and possibly the handles on the
 // moved-from object.