<webview>: Make contentWindow available prior to attachment (on display:none).

This CL updates the createGuest API to return an optional contentWindow.

This CL renames GuestSizer to GuestProxyHost which better represents what it does.

GuestProxyHost exposes "LoadURLWithParams" which, after navigating, creates a swapped out RenderView for the guest in the embedder's process.

This CL removes RegisterDestructionCallback from BrowserPluginGuestDelegate.

BUG=456118

Test=WebViewTest.NoContentSettingsAPI runs with display:none now.

Review URL: https://codereview.chromium.org/910073003

Cr-Commit-Position: refs/heads/master@{#319833}
20 files changed