Move didFocus from WebWidgetClient to WebViewClient.

This method is used to bring the window to focus, and only makes sense
when called on the main frame.

This is part of a refactor to split WebView/WebWidget.

BUG=419087

Review-Url: https://codereview.chromium.org/2157113002
Cr-Commit-Position: refs/heads/master@{#406609}
10 files changed