Add the <t> element and ignore whitespace outside it.

Add the <t> element and ignore whitespace outside it.

We now only preserve the whitespace inside a <t> element inside
the parser. This removes the known n^2 from reattaching whitespace
which should make parsing and appending nodes faster. I also
removed the dead WhitespaceMode code from the parser, and made
the dom-seralizer.sky auto indent the markup so the test output
would be readable.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/a19a04db3d662adac5707e9f62e8fae098b327b7

[abarth-chromium, 2015-01-24 00:28:30]

lgtm

[abarth-chromium, 2015-01-24 00:46:47]

lgtm

[esprehn, 2015-01-24 00:50:07]

Committed patchset #3 (id:2) manually as
a19a04db3d662adac5707e9f62e8fae098b327b7 (presubmit successful).