Bug 1262439 - 1 - Introduce a new eye-dropper highlighter; r=zer0

This is mostly porting code from the XUL eye-dropper over to a new
highlighter file.
While I have tested this locally, this highlighter isn't yet used
in devtools.
This will come in later patches.

MozReview-Commit-ID: IF0puLu5Yc7

devtools/shared/layout/utils.js

@@ -166,6 +166,7 @@ function getFrameOffsets(boundaryWindow, node) {
 
   return [xOffset * scale, yOffset * scale];
 }
+exports.getFrameOffsets = getFrameOffsets;
 
 /**
  * Get box quads adjusted for iframes and zoom level.