Re-land "Finish ReadBuffer support in end2end_tests."

Currently we had a few TODOs concerned about unimplemented calls in
ANGLE. ANGLE now has support for ReadBuffer, so uncomment the calls.

Re-land with end2end_tests fix.

BUG=angleproject:940

Change-Id: I6e00c3aac6307308fb7f00c7e0426812563ed778
Reviewed-on: https://chromium-review.googlesource.com/317821
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/tests/gl_tests/ClearTest.cpp b/src/tests/gl_tests/ClearTest.cpp
index 5062bdc..518599a 100644
--- a/src/tests/gl_tests/ClearTest.cpp
+++ b/src/tests/gl_tests/ClearTest.cpp
@@ -177,11 +177,9 @@
     ASSERT_GL_NO_ERROR();
     EXPECT_PIXEL_EQ(0, 0, 255, 255, 255, 255);
 
-    // TODO: glReadBuffer support
-    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, 0, 0);
-    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, textures[1], 0);
+    glReadBuffer(GL_COLOR_ATTACHMENT1);
+    ASSERT_GL_NO_ERROR();
 
-    //TODO(jmadill): Robust handling of pixel test error ranges
     EXPECT_PIXEL_NEAR(0, 0, 0, 127, 255, 255, 1);
 
     glDeleteTextures(2, textures);