PPB_Flash.Navigate(): Disallow certain HTTP request headers.

With this CL, PPB_Flash.Navigate() fails the operation with
PP_ERROR_NOACCESS if the request headers contain non-simple headers.

BUG=332023
TEST=None

Review URL: https://codereview.chromium.org/136393004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249114 0039d316-1c4b-4281-b951-d872f2087c98