[Vulkan] Enable kRGB_888x_SkColorType in SkColorTypeToVkFormat()

Previously, gfx::SkColorTypeToVkFormat() wasn't mapping
kRGB_888x_SkColorType because VK_FORMAT_R8G8B8_UNORM was not supported
by Skia, but it should be able to handle that image format now.

Bug: 861853
Change-Id: Ic2b861f42628946484efb872aaf0c43aad5d660b
Reviewed-on: https://chromium-review.googlesource.com/c/1372724Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615808}

ui/gfx/skia_util.cc

@@ -170,8 +170,8 @@ VkFormat SkColorTypeToVkFormat(SkColorType color_type) {
       return VK_FORMAT_B4G4R4A4_UNORM_PACK16;
     case kRGBA_8888_SkColorType:
       return VK_FORMAT_R8G8B8A8_UNORM;  // or VK_FORMAT_R8G8B8A8_SRGB
-    case kRGB_888x_SkColorType:  // Skia doesn't support it yet.
-      break;
+    case kRGB_888x_SkColorType:
+      return VK_FORMAT_R8G8B8_UNORM;
     case kBGRA_8888_SkColorType:
       return VK_FORMAT_B8G8R8A8_UNORM;  // or VK_FORMAT_B8G8R8A8_SRGB
     case kRGBA_1010102_SkColorType: