Fix differences between chrome and ff

As described in https://github.com/topcoat/topcoat/issues/388
Firefox parity issue

Added a new variable `var-background-image-pos--search-input = .6rem center`
to center the background image for the search-input, also added height property
equal to the line-height to fix dimensions issue in firefox

reported : https://github.com/topcoat/topcoat/issues/388
Removed variable using value instead

Removed the background-position variable because it was the same
for both desktop and mobile.
Merge pull request #9 from topcoat/ff-parity

Fix differences between chrome and ff