Expose an app_targets variable similar to android_app_targets

This target provides Chromium embedders with an elegant way to include
additional targets for all platforms.

BUG=453606

Review URL: https://codereview.chromium.org/882253007

Cr-Original-Commit-Position: refs/heads/master@{#313831}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 25c9c6c9b35147b4d3e9bababade01d9fe444968

all.gyp

@@ -5,7 +5,9 @@
 {
   'variables': {
     # A hook that can be overridden in other repositories to add additional
-    # compilation targets to 'All'. Only used on Android.
+    # compilation targets to 'All'.
+    'app_targets%': [],
+    # For Android-specific targets.
     'android_app_targets%': [],
   },
   'targets': [
@@ -14,6 +16,7 @@
       'type': 'none',
       'xcode_create_dependents_test_runner': 1,
       'dependencies': [
+        '<@(app_targets)',
         'some.gyp:*',
         '../base/base.gyp:*',
         '../components/components.gyp:*',