Add lots of testonly annotations to the GN build.

BUG=
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Review URL: https://codereview.chromium.org/547793002

Cr-Commit-Position: refs/heads/master@{#293774}
diff --git a/content/shell/BUILD.gn b/content/shell/BUILD.gn
index 8c372a9d..bad7f4d 100644
--- a/content/shell/BUILD.gn
+++ b/content/shell/BUILD.gn
@@ -20,6 +20,7 @@
 }
 
 static_library("content_shell_lib") {
+  testonly = true
   sources = [
     "android/shell_jni_registrar.cc",
     "android/shell_jni_registrar.h",
@@ -428,6 +429,7 @@
 }
 
 executable("content_shell") {
+  testonly = true
   # TODO(GYP) mac resource bundle stuff for this target.
   # TODO(GYP) Windows content shell settings:
   #  - Manifest.