Vulkan: Move RenderPass init into a helper function. Here it can be more easily accessed for command re-ordering, which will use a RenderPass cache instead of having RenderPasses be owned by the Framebuffer. Bug: angleproject:2264 Change-Id: I9b06cff43e536a526d44e7e0c04027bc450051cf Reviewed-on: https://chromium-review.googlesource.com/789533 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>