Rename ClientRect and ClientRectList to DOMRect and DOMRectList (fixes #2814, fixes #2840).

These interfaces were renamed in the specification, in order to use them in
other contexts than the getClientRects and getBoundingClientRect methods.
Merge pull request #3058 from Ms2ger/2814-DOMRect

Rename ClientRect and ClientRectList to DOMRect and DOMRectList (fixes #2814, fixes #2840); r=abinader