Fix limits tests with new extra limits

The code was not checking if the limits actually exist.
If they don't then the test would fail. Since these limits
have not been added to the spec, they're still being discussed,
they are not required and therefore the tests should still run
without them.

Author: greggman

src/webgpu/api/validation/capability_checks/limits/limit_utils.ts

@@ -417,10 +417,12 @@ export class LimitTestsImpl extends GPUTestBase {
     if (extraLimits) {
       for (const [extraLimitStr, limitMode] of Object.entries(extraLimits)) {
         const extraLimit = extraLimitStr as GPUSupportedLimit;
-        requiredLimits[extraLimit] =
-          limitMode === 'defaultLimit'
-            ? getDefaultLimitForAdapter(adapter, extraLimit)
-            : (adapter.limits[extraLimit] as number);
+        if (adapter.limits[extraLimit] !== undefined) {
+          requiredLimits[extraLimit] =
+            limitMode === 'defaultLimit'
+              ? getDefaultLimitForAdapter(adapter, extraLimit)
+              : (adapter.limits[extraLimit] as number);
+        }
       }
     }