Loosen some OpenXR validations

Running the OpenXR mock failed with the sandbox disabled because the
number of requested extensions did not match the number of supported
extensions. This isn't a strict requirement of the runtime, and really,
we should only care that all enabled extensions are actually supported,
which is validated by the next check.

Change-Id: I96b3961c886ff16642c3c3f2005e28e5a60b7b30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391216
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804418}

device/vr/openxr/test/fake_openxr_impl_api.cc

@@ -150,10 +150,6 @@ XrResult xrCreateInstance(const XrInstanceCreateInfo* create_info,
       XR_ERROR_VALIDATION_FAILURE,
       "XrInstanceCreateInfo ApiLayer is not supported by this version of test");
 
-  RETURN_IF(create_info->enabledExtensionCount !=
-                OpenXrTestHelper::kNumExtensionsSupported,
-            XR_ERROR_VALIDATION_FAILURE, "enabledExtensionCount invalid");
-
   for (uint32_t i = 0; i < create_info->enabledExtensionCount; i++) {
     bool valid_extension = false;
     for (size_t j = 0; j < OpenXrTestHelper::kNumExtensionsSupported; j++) {