Vulkan: Bump minImageCount for swap chain to match GL

GL uses triple buffering, but Vk was set to use double in most cases.
Bump Vk to triple buffering for now.

Change-Id: I8e68106b5f0306eb6867382d58d1195341d62a07
Reviewed-on: https://chromium-review.googlesource.com/c/1388223Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619978}

gpu/vulkan/vulkan_swap_chain.cc

@@ -210,7 +210,7 @@ bool VulkanSwapChain::InitializeSwapChain(
   swap_chain_create_info.sType = VK_STRUCTURE_TYPE_SWAPCHAIN_CREATE_INFO_KHR;
   swap_chain_create_info.surface = surface;
   swap_chain_create_info.minImageCount =
-      std::max(2u, surface_caps.minImageCount);
+      std::max(3u, surface_caps.minImageCount);
   swap_chain_create_info.imageFormat = surface_format.format;
   swap_chain_create_info.imageColorSpace = surface_format.colorSpace;
   swap_chain_create_info.imageExtent = surface_caps.currentExtent;