Add various GPU isolates for GN.

This adds support for some of test binaries needed in the
'archive_gpu_tests=true' config of a GYP build:

- content_gl_tests
- gles2_conform_tests
- gl_tests
- tab_capture_end2end_tests

There is still more work to be done (telemetry_gpu_tests,
angle_end2end_tests, angle_deqp_*_tests).

[email protected]
BUG=519778

Review URL: https://codereview.chromium.org/1285063002

Cr-Commit-Position: refs/heads/master@{#343095}
diff --git a/BUILD.gn b/BUILD.gn
index 3f5ab117..9492d51 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -91,6 +91,7 @@
       "//extensions:extensions_unittests",
       "//google_apis/gcm:gcm_unit_tests",
       "//gpu:gpu_unittests",
+      "//gpu/gles2_conform_support:gles2_conform_test",
       "//ipc:ipc_tests",
       "//ipc/mojo:ipc_mojo_unittests",
       "//jingle:jingle_unittests",