Section alignment corrections

* Used a div as placeholder instead of using the same tag in source as
  placeholer, so that we can better control the dimensions of this div
* Initiate the calculate the section dimensions only after a few milliseconds
  delay to make sure the style sheets of source page is ready and applied
* If the element is figure, adjust the alignment using figcaption and not with
  parent figure.

Bug: 66411
Change-Id: Ic71b52d2218e7ffe971d249a917a26b00f9afc33