Enable viewport tests on OpenGL ES.

Also implement a workaround for missing glViewportIndexedf().

BUG=dawn:580,dawn:597

Change-Id: I618161ae9750925c1c892123607db84563f0869c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/34721
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/end2end/ViewportOrientationTests.cpp

@@ -68,4 +68,5 @@ DAWN_INSTANTIATE_TEST(ViewportOrientationTests,
                       D3D12Backend(),
                       MetalBackend(),
                       OpenGLBackend(),
+                      OpenGLESBackend(),
                       VulkanBackend());

src/tests/end2end/ViewportTests.cpp

@@ -213,4 +213,5 @@ DAWN_INSTANTIATE_TEST(ViewportTest,
                       D3D12Backend(),
                       MetalBackend(),
                       OpenGLBackend(),
+                      OpenGLESBackend(),
                       VulkanBackend());