Add nonzero_clear_resources_on_creation_for_testing toggle to opengl

Forces texture to clear to nonzero on creation to test the logic of
lazy clearing.

Bug: dawn:145
Change-Id: Ib1f6e8f961927008e3c09960ad0219f975d86cc6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7240
Commit-Queue: Natasha Lee <natlee@microsoft.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/end2end/NonzeroTextureCreationTests.cpp

@@ -95,5 +95,7 @@ TEST_P(NonzeroTextureCreationTests, ArrayLayerClears) {
 }
 
 DAWN_INSTANTIATE_TEST(NonzeroTextureCreationTests,
+                      ForceWorkaround(OpenGLBackend,
+                                      "nonzero_clear_resources_on_creation_for_testing"),
                       ForceWorkaround(VulkanBackend,
                                       "nonzero_clear_resources_on_creation_for_testing"));