Allow lines empty but for tabs and comments to be ignored.

We originally allowed only spaces around comments.  Now allow tabs as
well.  This ought to affect very few pages, but it helps predictability
and to maintain consistency between the PHP preprocessor and parsoid.

Change-Id: Icb3ff6eec08aaa83ae332d03c910c13995c9c9ee
Merge "Allow lines empty but for tabs and comments to be ignored."