For the technically curious, this feature has been intentionally disabled on certain browsers.
There are some long-standing bugs in WebKit (
https://bugs.webkit.org/show_bug.cgi?id=23372,
https://bugs.webkit.org/show_bug.cgi?id=31253) that causes excessive memory allocation when the line drawing feature is enabled. The problem is so severe that it can lead to unbounded memory allocation and eventually cause the browser to crash. This affects all WebKit derived browsers including every flavor of Chrome (Mac/Win) and Safari (Mac/iOS/Win).
We disabled the line drawing feature on WebKit browsers so that everyone has at least a partially working tool as opposed to a crashed browser. If or when these bugs get fixed, we can re-enable this feature across all browsers.