Factor out uses of WebContentsModalDialog interface functions from WebContentsModalDialogManager

Factor out uses of WebContentsModalDialog interface functions from WebContentsModalDialogManager

This CL is part 3 of 6 in the work to remove the WebContentsModalDialog
interface.  It removes uses of the WebContentsModalDialog interface from
WebContentsModalDialogManager, in favor of delegating to the appropriate
NativeWebContentsModalDialogManager subclass.  This adds new subclasses
for Cocoa and Gtk to handle those platforms.

This builds on part 2 in https://codereview.chromium.org/12276010.

BUG=157161
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=184259

[Mike Wittman, 2013-02-16 00:51:18]

Ben, please take a look at the non-Cocoa parts of this.

Sailesh, please take a look at the Cocoa parts of this.

Elliot, please take a look at the Gtk parts of this.

[Ben Goodger (Google), 2013-02-16 00:58:27]

lgtm

[Elliot Glaysher, 2013-02-16 01:02:15]

gtk lgtm

[sail, 2013-02-16 21:52:00]

I'm not sure what NativeWebContentsModalDialogManager is but LGTM I guess.

[Mike Wittman, 2013-02-22 21:02:42]

On 2013/02/16 21:52:00, sail wrote:
> I'm not sure what NativeWebContentsModalDialogManager is but LGTM I guess.

FYI NativeWebContentsModalDialogManager supports moving to the opaque
NativeWebContentsModalDialog type by providing platform-specific operations on
web contents modal dialogs.  These operations are used by
WebContentsModalDialogManager in response to events like tab focused or closed.

[commit-bot: I haz the power, 2013-02-22 22:14:49]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wittman@chromium.org/12286014/8003

[commit-bot: I haz the power, 2013-02-23 02:23:05]

Change committed as 184259