Add a dummy //:chromium_builder_tests to the GN build.

Apparently some bots still use this target to build targets,
if they're not building a specific list of tests.

TBR=brettw@chromium.org, brucedawson@chromium.org
BUG=581899
NOTRY=true

Review URL: https://codereview.chromium.org/1900003002 .

Cr-Commit-Position: refs/heads/master@{#388083}

BUILD.gn

@@ -78,6 +78,16 @@ group("All") {
   ]
 }
 
+# TODO(GYP): This target exists for compatibility with GYP for the builders.
+# For now, this builds everything. We should decide if we want to build less.
+group("chromium_builder_tests") {
+  testonly = true
+
+  deps = [
+    ":All",
+  ]
+}
+
 # The "both_gn_and_gyp" target should reflect every target that is built
 # in both the GN and GYP builds, and ideally it should match the
 # "both_gn_and_gyp" target in build/gn_migration.gypi line-for-line.