Make WebView keep keyboard when losing focus

WebView should not hide keyboard when losing focus. As for Chrome, it
should keep current behavior. These cases include when the ContentView is
hidden (closing current tab, opening a new tab, switching to an existing
tab), and when imeAdapter is attached to its native counterpart(navigation).
Note that this CL will change one behavior of Chrome: keyboard will not be
hidden when focus moves from ContentView to UrlBar.

BUG=636237

Review-Url: https://codereview.chromium.org/2290133002
Cr-Commit-Position: refs/heads/master@{#429531}
7 files changed