Fix memory leak when creating SkBitmapProcShader

A SkShader is ref counted, but the first but unref() was
never called for the local reference in
PlatformGraphicsContextSkia::drawBitmapPattern().

Change-Id: Ia1936d62f052f9440ef0316cc642b0aebd79cfe1
Signed-off-by: DHO <DHO@dho.im>