Port custom_tabs_client from GYP to GN

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 06f6d0f8840a5cda3e2bf640da6a8d62739e0a37..1f7a7c4d2fb86c04220a6e3430e31f6e07acc851 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -221,6 +221,7 @@ group("both_gn_and_gyp") {
       "//chrome/android:chrome_public_apk",
       "//chrome/android:chrome_public_test_apk",
       "//chrome/test/chromedriver/test/webview_shell:chromedriver_webview_shell_apk",
+      "//third_party/custom_tabs_client:custom_tabs_client_example_apk",

[pasko, 2015/09/29 09:38:58]

this puts custom_tabs_client_example_apk as a dependency to all android target
or I am reading it wrong?

The GYP target does not put itself anywhere except the All target, and also
conditionalized on customtabs==0.

I thought we trying to mirror the logic with GYP, then I am lacking context on
why these particular bits were decided not to be replicated.

Please educate me :)

       "//third_party/errorprone:chromium_errorprone",
       "//tools/android:android_tools",
       "//tools/imagediff($host_toolchain)",