Add blimp to root GN check_targets

content/app/BUILD.gn

Index: content/app/BUILD.gn
diff --git a/content/app/BUILD.gn b/content/app/BUILD.gn
index 8663cf89ef3c4ac0e628392943264c3c9518ea8f..00381f97e4ea4eb9f1ae0061df94da81d4999fd5 100644
--- a/content/app/BUILD.gn
+++ b/content/app/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
+import("//build/config/features.gni")
 
 content_app_sources = [
   "android/app_jni_registrar.cc",
@@ -52,6 +53,10 @@ if (is_win) {
   ]
 }
 
+if (enable_plugins) {

[Dirk Pranke, 2016/04/28 00:25:40]

For posterity, I think in order to address brettw's comments in #59 and #60, I
think we should change this to if (is_linux && enable_plugins), and not add the
dependency from blimp to //content/public/child. I think if we do what I
suggest, we keep the dependencies in blimp "right" and make content/app
"slightly more right", whereas if we do brettw's suggestion in #60, we make
blimp more confusing and don't make content any less confusing. 

[nyquist, 2016/04/28 00:56:00]

I implemented both... So, in patch set 17 below you'll find the latest and
greatest, which for now is your suggestion of adding is_linux.

+  content_app_deps += [ "//content/ppapi_plugin:ppapi_plugin_sources" ]
+}
+
 content_app_extra_configs = [
   "//build/config/compiler:wexit_time_destructors",
   "//content:content_implementation",