Prevent pages from being horizontally scrollable.

On pages with really wide images or tables, the page becomes annoyingly
scrollable horizontally, which ruins the experience of scrolling the page
up and down, when it starts drifting left and right.  This handles two
cases that can potentially make the page scrollable horizontally:

- Detects large divs that have an explicit width that is larger than the
  viewport width, and sets their "overflow" style to "auto," so that the
  div itself becomes scrollable, instead of the entire page. Note: this
  only applies to API >10 (has no effect on API 10)
- Detects math formula images, which are often wider than the viewport
  width, and explicitly sets their width to fit within the viewport. The
  formulas will still be readable, but will simply appear "smaller."

Change-Id: I47e9b616c3cfc7f6f0f04f56f0f02be4da69e932