Handle GrContext::MakeVulkan() failures in VulkanInProcessContextProvider

GrContext::MakeVulkan() may fail and return nullptr.
VulkanInProcessContextProvider::Initialize() wasn't handling this case
properly. Fixed it to return false in that case. Also updated
RasterDecoderContextState to DCHECK that the vk_context_provider is
properly initialize (i.e. gr_context != nullptr).

Bug: 920000
Change-Id: If20df461d63b596210fb077c19919f7b3504e9cd
Reviewed-on: https://chromium-review.googlesource.com/c/1404334
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621899}
2 files changed