Fix cue rendering test and include support for left/right alignment.

The test has been broken for a while somehow, but only because at some
point the code for computing the width got close to the current version
of the WebVTT specification and the alignment has an impact on the width
of the text (so not specifying the alignment determines the default
align:middle and in some cases the width gets zero, e.g. with position:100%)

The specification also got changed in the sense that it also supports
left and right values for cue alignment, which are now included in the test
case and supported in the code.

TEST=Update existing test case.
BUG=299752
R=acolwell,scherkus

Review URL: https://codereview.chromium.org/25155003

git-svn-id: svn://svn.chromium.org/blink/trunk@159228 bbb929c8-8fbe-4397-9dbb-9b2b20218538