Web compat analysis:
https://groups.google.com/d/msg/mozilla.dev.platform/42-W463M4Ig/TwrqPyCjAAAJ

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/00923f90277216a21263f16898fb404c4a5741c8

commit 00923f90277216a21263f16898fb404c4a5741c8
Author: fs <fs@opera.com>
Date: Wed May 11 13:54:41 2016

Change "invalid value default" for HTMLTrackElement 'kind' to "metadata"

Rename TrackBase::defaultKind() to invalidValueDefaultKind() to better
reflect its semantics. Also make sure that the "missing value default"
is set appropriately (in TextTrack constructors and on removal in
HTMLTrackElement parseAttribute) now that it differs from the "invalid
value default".

The test media/track/track-kind.html is adjusted so that it doesn't
check if a cue is displayed, since that depends on unspecified behavior
wrt how 'mode' changes when 'kind' does. (See comment in
TextTrack::setKind.)

The WPT tests will eventually get updated via Mozilla's automatic sync,
so adding expectations for now.
(Ref: https://bugzilla.mozilla.org/show_bug.cgi?id=1269712)

Intent: https://groups.google.com/a/chromium.org/d/topic/blink-dev/6-oPQN4lZ2o/discussion

https://github.com/whatwg/html/issues/293
https://html.spec.whatwg.org/multipage/embedded-content.html#attr-track-kind

BUG= 608772 

Review-Url: https://codereview.chromium.org/1947033002
Cr-Commit-Position: refs/heads/master@{#392911}

[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/fast/dom/element-attribute-js-null-expected.txt
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/fast/dom/element-attribute-js-null.html
[add] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/imported/web-platform-tests/html/semantics/embedded-content/media-elements/interfaces/HTMLElement/HTMLTrackElement/kind-expected.txt
[add] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/imported/web-platform-tests/html/semantics/embedded-content/media-elements/interfaces/TextTrack/kind-expected.txt
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/media/track/opera/interfaces/HTMLElement/HTMLTrackElement/kind.html
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/media/track/opera/interfaces/TextTrack/kind.html
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/LayoutTests/media/track/track-kind.html
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/HTMLTrackElement.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/AudioTrack.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/AudioTrack.h
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/InbandTextTrack.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/TextTrack.h
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/TrackBase.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/TrackBase.h
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/VideoTrack.cpp
[modify] https://crrev.com/00923f90277216a21263f16898fb404c4a5741c8/third_party/WebKit/Source/core/html/track/VideoTrack.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338

commit 80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338
Author: fs <fs@opera.com>
Date: Mon May 16 09:05:07 2016

Clean up HTMLTrackElement.kind invalid/missing value default handling

Get rid of the isValidKind(...) and invalidValueDefaultKind() virtual
methods on TrackBase and do any required checking "up front" instead
as required. This should present less surprises and work in a less
side-effectful way.

Also start setting the 'kind' directly in the constructor rather than
invoking setKind() in (all) the constructor body (bodies).
Drop some redundant parenthesis and fix some obviously "wrong" names
in TextTrack.cpp.

BUG= 608772 

Review-Url: https://codereview.chromium.org/1973343002
Cr-Commit-Position: refs/heads/master@{#393812}

[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/HTMLTrackElement.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/AudioTrack.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/AudioTrack.h
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/InbandTextTrack.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/TextTrack.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/TextTrack.h
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/TrackBase.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/TrackBase.h
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/VideoTrack.cpp
[modify] https://crrev.com/80ef82107b8f9c2cd1bc707d86bc0fd8bc8cd338/third_party/WebKit/Source/core/html/track/VideoTrack.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aea239459e30190050f8897b6e5b2b9856d2de4f

commit aea239459e30190050f8897b6e5b2b9856d2de4f
Author: fs <fs@opera.com>
Date: Tue May 17 14:38:04 2016

Remove mode-transition in TextTrack::setKind

This mode-transition gives rise to inconsistent state-updates, and isn't
specified [1]. Remove it, and with it the setKind override.

Also move all the mutators to TextTrack since that's the only type of
track where these fields can be mutated (and only if sourced via a
HTMLTrackElement.)

[1] https://html.spec.whatwg.org/multipage/embedded-content.html#attr-track-kind

BUG= 460923 , 608772 

Review-Url: https://codereview.chromium.org/1984663002
Cr-Commit-Position: refs/heads/master@{#394122}

[modify] https://crrev.com/aea239459e30190050f8897b6e5b2b9856d2de4f/third_party/WebKit/Source/core/html/track/TextTrack.cpp
[modify] https://crrev.com/aea239459e30190050f8897b6e5b2b9856d2de4f/third_party/WebKit/Source/core/html/track/TextTrack.h
[modify] https://crrev.com/aea239459e30190050f8897b6e5b2b9856d2de4f/third_party/WebKit/Source/core/html/track/TrackBase.cpp
[modify] https://crrev.com/aea239459e30190050f8897b6e5b2b9856d2de4f/third_party/WebKit/Source/core/html/track/TrackBase.h

Renamed Blink>Text>Track to Blink>Media>Track. Moving these bugs to the new component.