recipes: add ndk_next gclient config.

This adds an ndk_next gclient configuration to allow bots to use the
upcoming version of the Android NDK.

BUG=668202

Change-Id: I403e398094f4215cdb3783b68f465323264ca1ee
Reviewed-on: https://chromium-review.googlesource.com/414184
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Mike Stipicevic <stip@chromium.org>

recipe_modules/gclient/config.py

@@ -591,3 +591,8 @@ def chromedriver(c):
   c.solutions[0].custom_deps[
       'src/chrome/test/chromedriver/third_party/java_tests'] = (
           ChromiumGitURL(c, 'chromium', 'deps', 'webdriver'))
+
+@config_ctx()
+def ndk_next(c):
+  c.revisions['src/third_party/android_tools/ndk'] = 'origin/next'
+

recipe_modules/gclient/example.py

@@ -42,6 +42,7 @@ TEST_CONFIGS = [
   'master_deps',
   'mojo',
   'nacl',
+  'ndk_next',
   'pdfium',
   'perf',
   'recipes_py',