DevTools: Properly prepend cm- to all classes that CodeMirror returns

Sometimes codemirror returns classes like "class1 class2", but only
the first one would be prepended with "cm-".

Bug: 955550
Change-Id: I6733c67e3a9074bc3566d356320b67f0e011c7ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1690743
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675927}