Metal: Test offline-compiled shader link time

Change metal_util::TestShader to test either compile or link, and
change the default behavior to link.

Add inline data for a trivial test shader. Add code to modify the
bitcode, to defeat caching.

Ensure that the Metal calls for the test are invoked on a thread
that has base::TaskPriority::HIGHEST.

TBR=jvanverth

Bug: 974219
Change-Id: I35e688c3758af2e03f893a1ccbef7aef6b778b23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015486Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734645}

components/metal_util/test_shader.h

@@ -12,6 +12,13 @@
 
 namespace metal {
 
+enum class METAL_UTIL_EXPORT TestShaderComponent {
+  // Test a shader compile from source.
+  kCompile,
+  // Test linking a precompiled shader.
+  kLink,
+};
+
 enum class METAL_UTIL_EXPORT TestShaderResult {
   // Not attempted (e.g, because macOS version does not support Metal).
   kNotAttempted,
@@ -37,7 +44,7 @@ constexpr base::TimeDelta kTestShaderTimeForever =
     base::TimeDelta::FromMinutes(3);
 
 // A default delay before attempting to compile the test shader.
-constexpr base::TimeDelta kTestShaderDelay = base::TimeDelta::FromSeconds(90);
+constexpr base::TimeDelta kTestShaderDelay = base::TimeDelta::FromMinutes(3);
 
 // Attempt to asynchronously compile a trivial Metal shader. If |delay| is zero,
 // then compile synchronously, otherwise, post a delayed task to do the compile.
@@ -51,7 +58,8 @@ constexpr base::TimeDelta kTestShaderDelay = base::TimeDelta::FromSeconds(90);
 void METAL_UTIL_EXPORT
 TestShader(TestShaderCallback callback,
            const base::TimeDelta& delay = kTestShaderDelay,
-           const base::TimeDelta& timeout = kTestShaderTimeout);
+           const base::TimeDelta& timeout = kTestShaderTimeout,
+           TestShaderComponent component = TestShaderComponent::kLink);
 
 }  // namespace metal