Add in a default case to unblock shaderc rolls

The roll in Chromium is failing, because Dawn doesn't handle all of the new enum
values, but I cannot land the 'proper' fix in Dawn, because Chromium doesn't yet
know about the new enum values. Yay, cross dependencies.

So my solution is land the default case in Dawn, land the DEPS roll in Chromium,
then patch in the 'proper' cases in Dawn.

Roll third_party/shaderc/ a7657e4fa..95185d920 (2 commits)

a7657e4fa2..95185d920a

$ git log a7657e4fa..95185d920 --date=short --no-merges --format='%ad %ae %s'
2020-03-17 rharrison Rolling 4 dependencies (#1002)
2020-03-16 rharrison Add reflection support for storage textures (#1001)

Created with:
  roll-dep third_party/shaderc

Change-Id: Id73b38643d9a6d418be9c75e218fdad7a18839bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/17164
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

DEPS

@@ -69,7 +69,7 @@ deps = {
     'condition': 'dawn_standalone',
   },
   'third_party/shaderc': {
-    'url': '{chromium_git}/external/github.com/google/shaderc@a7657e4fa2feca67e535bfb20d4fa89fd704921e',
+    'url': '{chromium_git}/external/github.com/google/shaderc@95185d920a0b3d5a493f6f705ad8f0245c7d55cf',
     'condition': 'dawn_standalone',
   },
 

src/dawn_native/ShaderModule.cpp

@@ -118,6 +118,11 @@ namespace dawn_native {
                 // textures when they are supported as shaderc_spvc binding types.
                 case shaderc_spvc_binding_type_storage_texture:
                     return wgpu::BindingType::StorageTexture;
+                default:
+                    // TODO(rharrison): Remove this case once I am done changing the
+                    // values in shaderc_spvc_binding_type
+                    UNREACHABLE();
+                    return wgpu::BindingType::StorageTexture;
             }
             UNREACHABLE();
         }