Implement AwPermissionManager::RequestPermissions

This is needed to work with new Web MIDI permission requests.

BUG=582328
TEST=git cl try
TEST=run_android_webview_unittests --gtest_filter='AwPermissionManagerTest.*'
TEST=run_system_webview_shell_layout_test_apk with built SystemWebViewGoogle.apk

Review-Url: https://codereview.chromium.org/2143573002
Cr-Commit-Position: refs/heads/master@{#408371}
5 files changed