Android: Add ndk path for generate_gradle.py

This way Android Studio makes use of our existing android ndk rather
than having to download a whole new ndk in third_party/android_tools.

Bug: 840542
Change-Id: Idcf06675f5e5f74cd860570102c38ae20dd97807
Reviewed-on: https://chromium-review.googlesource.com/1054439Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558305}

build/android/BUILD.gn

@@ -56,6 +56,8 @@ if (enable_java_templates) {
   _data +=
       "android_sdk_root=" + rebase_path(android_sdk_root, root_build_dir) + CR
   _data += "android_sdk_version=$android_sdk_version$CR"
+  _data +=
+      "android_ndk_root=" + rebase_path(android_ndk_root, root_build_dir) + CR
   _data += "android_tool_prefix=" +
            rebase_path(android_tool_prefix, root_build_dir) + CR
   write_file(android_build_vars, _data)

build/android/gradle/generate_gradle.py

@@ -535,10 +535,11 @@ def _CreateJniLibsDir(output_dir, entry_output_dir, so_files):
   return []
 
 
-def _GenerateLocalProperties(sdk_dir):
+def _GenerateLocalProperties(sdk_dir, ndk_dir):
   """Returns the data for project.properties as a string."""
   return '\n'.join([
       '# Generated by //build/android/gradle/generate_gradle.py',
+      'ndk.dir=%s' % ndk_dir,
       'sdk.dir=%s' % sdk_dir,
       ''])
 
@@ -941,8 +942,9 @@ def main():
       sdk_path = os.path.expanduser('~/Android/Sdk')
     else:
       sdk_path = _RebasePath(build_vars['android_sdk_root'])
+    ndk_path = _RebasePath(build_vars['android_ndk_root'])
     _WriteFile(os.path.join(generator.project_dir, 'local.properties'),
-               _GenerateLocalProperties(sdk_path))
+               _GenerateLocalProperties(sdk_path, ndk_path))
 
   if args.full:
     zip_tuples = []