[directorytree] Renaming controller: add a getLabelElement helper

Move the .label CSS class element querying code into a helper routine,
and use that helper everywhere in this code.

Covered by existing directory tree renaming tests.

Tbr: lucmult
Bug: 984428
Change-Id: I0b6ec9f35af259f75fd2339242b3ac1a082db03c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703028
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677640}
1 file changed