Dawn: use Dawn's temporary test targets for isolates.

The plan is to move the actual test targets in Dawn to Dawn's
src/test directory. This change makes Chrome temporarily point at
some intermediate proxy targets, so we can move the real ones
without breaking Chrome.

Bug: dawn:61
Change-Id: I166c17f1689e2a2e71b1607af44f6f9a8c14b6af
Reviewed-on: https://chromium-review.googlesource.com/c/1372709Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615715}

testing/buildbot/gn_isolate_map.pyl

@@ -546,7 +546,7 @@
     "type": "console_test_launcher",
   },
   "dawn_end2end_tests": {
-    "label": "//third_party/dawn:dawn_end2end_tests",
+    "label": "//third_party/dawn:dawn_end2end_tests_temp_group",
     "type": "raw",
     "args": [],
   },