Add post content type for form submission

[chromium-wpt-export-bot]

The form submission with POST, target '_blank', no opener is handled as browser-initiated. So BeginNavigationParams should consider the content-type.

This CL make the content type to be added to BeginNavigationParams which is created in NavigationRequest::CreateBrowserInitiated().

Fixed: 1343390, 1343609 Change-Id: I9e663d3cf16cf7a6ab989c72b728ea31fc75fd3f Reviewed-on: https://chromium-review.googlesource.com/3754790 WPT-Export-Revision: ce000f34985e30f57442d1e0c1d3a6444d58194a