command_buffer: Implement glGetStringi for GL ES 3.0

Implement glGetStringi for GL ES 3.0.
It is available for GL ES 2.0 contexts, too.

Skia uses the function to get extensions on ES 3.0.

BUG=581634
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel

Review URL: https://codereview.chromium.org/1688163002

Cr-Commit-Position: refs/heads/master@{#376462}
diff --git a/gpu/BUILD.gn b/gpu/BUILD.gn
index 0bf669a4..c3f82eb 100644
--- a/gpu/BUILD.gn
+++ b/gpu/BUILD.gn
@@ -166,6 +166,7 @@
     "command_buffer/tests/gl_program_unittest.cc",
     "command_buffer/tests/gl_query_unittest.cc",
     "command_buffer/tests/gl_readback_unittest.cc",
+    "command_buffer/tests/gl_request_extension_unittest.cc",
     "command_buffer/tests/gl_shared_resources_unittest.cc",
     "command_buffer/tests/gl_stream_draw_unittest.cc",
     "command_buffer/tests/gl_test_utils.cc",