Android Studio: Run `gn gen` if necessary

Specifically when passing in targets manually. The `--all` flag (on by
default) already takes care of this case.

Bug: 620034
Change-Id: Ie5c679c6cf9b75e36415d2868f7fff46e9e81ad1
Reviewed-on: https://chromium-review.googlesource.com/1089121
Commit-Queue: Peter Wen <wnwen@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564979}

build/android/gradle/generate_gradle.py

@@ -876,6 +876,9 @@ def main():
     assert not args.native_targets, 'Native editing requires --all.'
     targets = [re.sub(r'_test_apk$', '_test_apk__apk', t)
                for t in targets_from_args]
+    # Necessary after "gn clean"
+    if not os.path.exists(os.path.join(output_dir, 'build_vars.txt')):
+      _RunGnGen(output_dir)
 
   build_vars = _ReadPropertiesFile(os.path.join(output_dir, 'build_vars.txt'))
   jinja_processor = jinja_template.JinjaProcessor(_FILE_DIR)