Revert "Convert spvc build flag to a runtime toggle"

This reverts commit 1954436fe2.

Reason for revert:
The shaderc side of this multi-patch brings in a dependency on glslang, which uses a static initializer, which chromium doesn't like.

Original change's description:
> Convert spvc build flag to a runtime toggle
> 
> Also moves some of the spirv_cross code into the main library that was
> feature guarded, since spvc requires it.
> 
> BUG=dawn:281
> 
> Change-Id: I482d1d5a5c851956d3815bad90665c52a1ea15bb
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13860
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>

TBR=cwallez@chromium.org,kainino@chromium.org,enga@chromium.org,rharrison@chromium.org

Change-Id: Ia9a025fb4440c96874d1b45776a9f97023ca591d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: dawn:281
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13941
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

src/dawn_native/Toggles.cpp

@@ -82,11 +82,7 @@ namespace dawn_native {
                "versions of Windows prior to build 1809, or when this toggle is turned off, Dawn "
                "will emulate a render pass."}},
              {Toggle::SkipValidation,
-              {"skip_validation", "Skip expensive validation of Dawn commands."}},
-             {Toggle::UseSpvc,
-              {"use_spvc",
-               "Enable use of spvc for shader compilation, instead of accessing spirv_cross "
-               "directly."}}}};
+              {"skip_validation", "Skip expensive validation of Dawn commands."}}}};
     }  // anonymous namespace
 
     void TogglesSet::SetToggle(Toggle toggle, bool enabled) {

src/dawn_native/Toggles.h

@@ -33,7 +33,7 @@ namespace dawn_native {
         UseD3D12ResourceHeapTier2,
         UseD3D12RenderPass,
         SkipValidation,
-        UseSpvc,
+
         EnumCount,
         InvalidEnum = EnumCount,
     };