vda_unittest: Use SCANOUT_VDA_WRITE when allocating on ChromeOS

We need to allocate the native pixmap for the unittest using the
SCANOUT_VDA_WRITE usage flag like we do in Chrome so we get the
right tiling.

Bug: 845076
Test: video_decode_accelerator_unittest passes
Change-Id: I877bf94e6f7f90c14973be76c1f53d841aed4305
Reviewed-on: https://chromium-review.googlesource.com/1072638Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562031}

media/gpu/test/video_decode_accelerator_unittest_helpers.cc

@@ -101,7 +101,8 @@ scoped_refptr<TextureRef> TextureRef::CreatePreallocated(
       VideoPixelFormatToGfxBufferFormat(pixel_format);
   texture_ref->pixmap_ =
       factory->CreateNativePixmap(gfx::kNullAcceleratedWidget, size,
-                                  buffer_format, gfx::BufferUsage::SCANOUT);
+                                  buffer_format,
+                                  gfx::BufferUsage::SCANOUT_VDA_WRITE);
   LOG_ASSERT(texture_ref->pixmap_);
 #endif