Improved p-wrapping in the presence of comments-and-ws-only lines.

* Lines that contained comments and white-text only used to trigger
  p-wrapping based on newlines seen till that point.  However, the
  PHP parser effectively discards such lines (including the newline)
  which leads to newlines before/afther such a line to be considered
  as sequential.

* This patch mimics the PHP behavior by effectively ignoring the
  comments-and-ws-only lines (but not discarding them since they
  still need to be roundtripped back).

* In addition, changed newline output in p-br-p case to emit the
  3rd newline after the <br/>. To account for this, modified:
  - DSR computation to allocate zero-dsr width to the br tag
    since the newline after it will account for it.
  - serializer br handler to set mix/max constraints to 2 newlines
    instead of 3 before since the emitted newline will ensure that
    the 3rd newline is output.  To be verified if this is the
    right fix.

* This now fixes parsing (and thus roundtripping of the following
  test snippets):
  1. "a\n\n   <!--foo-->\n\nb"
  2. "a\n\n   \n\nb"
  3. "a\n\n\n=b="
  4. "a\n<!--foo-->\n\n\nb"
  5. "a\n\n<!--foo-->\n\nb"
  6. "a\n\n\n<!--foo-->\nb"

  Additional tests:
  1. "a\n<!--foo-->\nb" vs "a\n<!--foo--><!--bar-->\nb"
     The latter doesn't rt correctly, but the parse output now
     mimics the php parser idiosynrasy of being dependent on
     # of comments on a line.

* Eliminates all semantic RT errors on:
  - en:Ayat al-Kursi (also zero rt errors)
  - en:West Yorkshire abbeys and priories (also zero rt errors)
  - en:St._Louis_County_Road_91

* No change in parser tests results, but some change in HTML output
  seen in failed tests (and possibly passed tests).

* TODO: Add the above and other parser tests to parserTests.txt

Change-Id: I51f0ee39dcf7e7a7e2800117a0711f4a4659d4ec