Guard checkout_android gclient check w/ build_with_chromiuim.

Un-breaking WebRTC to let them revert a CL to un-break CrOS.

TBR=agrieve@chromium.org

Bug: 836344
Change-Id: I48abc7353825571873f63f37af54026db6c429c8
Reviewed-on: https://chromium-review.googlesource.com/1028571Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553822}

build/config/android/config.gni

@@ -5,14 +5,19 @@
 # This file contains common system config stuff for the Android build.
 
 if (is_android) {
-  import("//build/config/gclient_args.gni")
   import("//build_overrides/build.gni")
   import("abi.gni")
 
+  if (build_with_chromium) {
+    # Some non-chromium projects (e.g. WebRTC) use our build configs
+    # heavily but don't write gclient args files.
+
+    import("//build/config/gclient_args.gni")
     if (defined(checkout_android)) {
       assert(checkout_android,
              "target_os in your .gclient configuration must include android.")
     }
+  }
 
   has_chrome_android_internal =
       exec_script("//build/dir_exists.py",