Support double tap in the Browser.

Support double tap in the Browser.

Add api to get the left edge of the block from the current (x,y). The code
was copied from Cary's change.
Todo:
1.need some tuning as we can see from nytimes.com that some times the left
  edge is not correct.
2.currently nav cache is not up to date while loading. This means the left
  edge may not be correct during loading.
3.if (x,y) is over an edit text box, or image, it should return the left
  edge of it. Currently it is not working as expected.

Added the code to store the extra scale factor, so that back/forward history
works correctly.