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>

scripts/dawn_features.gni

@@ -44,6 +44,10 @@ declare_args() {
   # GLSL/HLSL/MSL compiler. This implicitly pulls in the GLSL
   # compiler, since it is a sub-class of if.
   dawn_enable_cross_reflection = false
+
+  # Enables using spvc for accessing SPIR-V toolchain, instead of
+  # directly accessing it in Dawn.
+  dawn_use_spvc = false
 }
 
 # GN does not allow reading a variable defined in the same declare_args().