Skip SharedImageGLBackingProduceDawnTest on Win7

Dawn is not supported on Win7. The tests were inadvertently enabled
in https://chromium-review.googlesource.com/c/chromium/src/+/2480584
which changed test set up. Now, the TestGPUServiceHolder is not
created at all on unsupported platforms. Use it as a signal to skip
the test.

Tbr: cwallez@chromium.org
Bug: 1139941
Change-Id: I68b4fcf733ca83978b544f7d065866760fc056db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485674Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818542}

gpu/command_buffer/tests/shared_image_gl_backing_produce_dawn_unittest.cc

@@ -74,7 +74,9 @@ class SharedImageGLBackingProduceDawnTest : public WebGPUTest {
   bool ShouldSkipTest() {
 // Windows is the only platform enabled passthrough in this test.
 #if defined(OS_WIN)
-    return false;
+    // Skip the test if there is no GPU service holder. It is not created if
+    // Dawn is not supported on the platform (Win7).
+    return GetGpuServiceHolder() == nullptr;
 #else
     return true;
 #endif  // defined(OS_WIN)