[MSE] Add support for SourceBuffer.mode.
https://bugs.webkit.org/show_bug.cgi?id=139265

Reviewed by Eric Carlson.

Source/WebCore:

Support setting SourceBuffer's mode property, and add support for "sequence" mode. Prior
to this change, "segments" mode was implied.

* Modules/mediasource/MediaSource.cpp:
(WebCore::MediaSource::addSourceBuffer):
(WebCore::MediaSource::regenerateActiveSourceBuffers):
* Modules/mediasource/SourceBuffer.cpp:
(WebCore::SourceBuffer::segmentsKeyword):
(WebCore::SourceBuffer::sequenceKeyword):
(WebCore::SourceBuffer::SourceBuffer):
(WebCore::SourceBuffer::setTimestampOffset):
(WebCore::SourceBuffer::sourceBufferPrivateDidReceiveSample):
(WebCore::SourceBuffer::setMode):
* Modules/mediasource/SourceBuffer.h:
* Modules/mediasource/SourceBuffer.idl:

LayoutTests:

* http/tests/media/media-source/mediasource-util.js:
* platform/mac/TestExpectations:

Add an expected-failure; the test fails due to lack of support for a non-DOM-visible
algorithm variable unsupported by the Mac port.
* platform/mac/http/tests/media/media-source/mediasource-sourcebuffer-mode-expected.txt: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177225 268f45cc-cd09-0410-ab3c-d52691b4dbfc