GitLab

- Adds ui.mojom.CompositionUnderline and ui.mojom.CompositionText.
- Adds corresponding struct traits + unittest for struct traits.
- Breaks ui.mojom.TextInputClient::OnCompositionEvent() into multiple
  functions, each corresponding to a function in ui::TextInputClient.

BUG=631524

Review-Url: https://codereview.chromium.org/2539453002
Cr-Commit-Position: refs/heads/master@{#436143}