sync compositor: pass simple gfx types by const ref

See bug for reasoning

BUG=159273

Review URL: https://codereview.chromium.org/1417893006

Cr-Commit-Position: refs/heads/master@{#356653}
void BrowserViewRenderer::ForceFakeCompositeSW() {
  DCHECK(compositor_);
  SkBitmap bitmap;
  bitmap.allocN32Pixels(1, 1);
  bitmap.eraseColor(0);
  SkCanvas canvas(bitmap);
  CompositeSW(&canvas);
}

void BrowserViewRenderer::ForceFakeCompositeSW() {
  DCHECK(compositor_);
  SkBitmap bitmap;
  bitmap.allocN32Pixels(1, 1);
  bitmap.eraseColor(0);
  SkCanvas canvas(bitmap);
  CompositeSW(&canvas);
}
