Dawn: use Dawn's temporary test targets.

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: I3c57af0dfbcff70b5acec30b69f2c013a7a9eb67
Reviewed-on: https://chromium-review.googlesource.com/c/1372320
Commit-Queue: Stephen White <senorblanco@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615647}

BUILD.gn

@@ -143,8 +143,8 @@ group("gn_all") {
       "//third_party/SPIRV-Tools/src:SPIRV-Tools",
       "//third_party/SPIRV-Tools/src/test/fuzzers",
       "//third_party/cacheinvalidation:cacheinvalidation_unittests",
-      "//third_party/dawn:dawn_end2end_tests",
-      "//third_party/dawn:dawn_unittests",
+      "//third_party/dawn:dawn_end2end_tests_temp_group",
+      "//third_party/dawn:dawn_unittests_temp_group",
       "//third_party/pdfium/samples:pdfium_test",
       "//third_party/webrtc/rtc_tools:frame_analyzer",
       "//tools/perf/clear_system_cache",