Add CacheRequest utilities and tests

This CL adds a DAWN_MAKE_CACHE_REQUEST X macro
which helps in building a CacheRequest struct.

A CacheRequest struct may be passed to LoadOrRun
which will generate a CacheKey from the struct and
load a result if there is a cache hit, or it will
call the provided cache miss function to compute a value.

The request struct helps enforce that precisely the
inputs that go into a computation are all also included
inside the CacheKey for that computation.

Bug: dawn:549
Change-Id: Id85eb95f1b944d5431f142162ffa9a384351be89
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91063
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn/tests/DawnNativeTest.h

diff --git a/src/dawn/tests/DawnNativeTest.h b/src/dawn/tests/DawnNativeTest.h
index e92bf67..dd3532f 100644
--- a/src/dawn/tests/DawnNativeTest.h
+++ b/src/dawn/tests/DawnNativeTest.h
@@ -38,12 +38,13 @@
     ~DawnNativeTest() override;
 
     void SetUp() override;
-    void TearDown() override;
 
+    virtual std::unique_ptr<dawn::platform::Platform> CreateTestPlatform();
     virtual WGPUDevice CreateTestDevice();
 
   protected:
     std::unique_ptr<dawn::native::Instance> instance;
+    std::unique_ptr<dawn::platform::Platform> platform;
     dawn::native::Adapter adapter;
     wgpu::Device device;