Add remaining GN targets for the Linux gpu tests.

This patch adds support for the telemetry-based
gpu tests to the GN build, adds a stubbed out
target for angle_end2end_tests (full support
depends on angle GN fixes), and adds a dummy
'chromium_swarm_tests' build target.

With these fixes, it should be possible to flip the
linux_chromium_rel_ng bot to GN and still have
everything run.

R=kbr@chromium.org, brettw@chromium.org
BUG=519778

Review URL: https://codereview.chromium.org/1311913004

Cr-Commit-Position: refs/heads/master@{#346067}

diff --git a/BUILD.gn b/BUILD.gn
index 3afc757..a3834a0 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -796,6 +796,10 @@
   }
 }
 
+# Add a dummy target for compatibility w/ GYP
+group("chromium_swarm_tests") {
+}
+
 if (is_linux) {
   # This group corresponds to the list of tests run on the waterfall for
   # desktop Linux GYP builds from testing/buildbot/chromium.linux.json. It's