Fix a merge error: re-add "track" attribute on TextTrackCue

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25263
trailing comma in enum SelectionMode

(cherry picked from commit 6af16dbb6b11ba3ba71a8696707ae78eaff62726)

Conflicts:
	source
make IDL conformance match reality for the time being
remove dialog, summary, details, showModalDialog, WindowModal, et al.
remove :dir() PC
remove seamless
isContentHandlerRegistered, isProtocolHandlerRegistered
removed input types datetime, datetime-local, week, month
removed reportValidity
remove style scoped
removed XMLDocument
add charset to 404
fix spurious x-data
fix number of columns in table
better, actual fixes for data-x and charset
fix links
CSSFONTS also informative
MEDIAFRAG is informative
[Bug 25528] Strip two residual mentions of srcset=''.
[e] (0) Try to elaborate on why the UA might pick particular media tracks
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24895
Affected topics: Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8543 340c8d12-0b0e-0410-8428-c7bf67bfef74
(cherry picked from commit c12bc2d5c2688c554cfd28805dd98610238900d6)
(cherry picked from commit ea7c2c9f1f90d6cd3e3f7296ac15bff9000834ea)