Fix VkFunctorDrawable crash

Fix WebView crash, which is specific for Vulkan pipeline
introduced by ag/8550137c.
VkFunctorDrawable expects SkDrawable::onSnapGpuDrawHandler
callback instead of SkDrawable::onDraw.
This CL invokes SkCanvas::drawDrawable/SkGpuDevice::drawDrawable,
which has the logic to invoke onSnapGpuDrawHandler.

Test: Don't crash WebView with Vulkan pipeline, pass CTS
Change-Id: Ia98f159511f4ad2dbdbe0d53f0aec2f8c6db263b