Unable to Extend Macintosh HD partition to use Free Space
Try booting from recovery or install media and following the same procedure. Usual caveat: make a full backup of things just in case.

I've had the same situation as you and it worked for me, the only difference was my drive was Time Machine rather than a boot volume. The route I took was make a bootable USB drive with DiskmakerX[1] rather then in-built recovery partition on the Mac.

[1] http://diskmakerx.com