tagId was incorrectly used as tsr info in TreeBuilderFixups

 * Populates data-stag with tag tsr info for later use when adding
   placeholder metas.

 * Fixes the long crashes we saw on round-trip tests. They were the
   result of not having a tsr end number on the meta tag, which soaked
   up the entire page into data-parsoid, subsequently dragging jsDiff to
   a halt.

Bug: 57069
Bug: 57071
Change-Id: Iff6341c5260b534282d13e4881aa38ea40722728