VFindMode: revert and use bounding client rects to calc width to fix a bug
VFindMode: revert and use bounding client rects to calc width to fix a bug

The previous `.scrollWidth` does not work on my laptop,
env:
- Windows 10 Education Insider x64 Chinese/build-14391.1002
- Chrome stable  55.0.2883.87 (64-bit)

The `scrollWidth` is the line width while `clientWidth` is the text width.

But on my PC, it works well (`scroll`->text; `client` -> line).

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus