Add basic supports of readonly and writeonly storage textures on Metal

This patch adds the basic supports of both read-only and write-only
storage textures on Metal with several simple end2end tests that use
read-only or write-only storage textures in every shader stage.

Here are the follow-ups after this patch:
1. test all the texture formats that can be used as both read-only and
write-only storage textures.
2. support using a texture with multiple different binding types in one
rendering or compute encoders.
3. test image2DArray, imageCube and imageCubeArray.

BUG=dawn:267
TEST=dawn_end2end_tests

Change-Id: Id0de623f7c48389b3b1e90b34a34fd16b14e1477
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19420
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

src/tests/DawnTest.cpp

@@ -532,6 +532,10 @@ bool DawnTestBase::IsSpvcBeingUsed() const {
     return gTestEnv->IsSpvcBeingUsed();
 }
 
+bool DawnTestBase::IsSpvcParserBeingUsed() const {
+    return gTestEnv->IsSpvcParserBeingUsed();
+}
+
 bool DawnTestBase::HasVendorIdFilter() const {
     return gTestEnv->HasVendorIdFilter();
 }