webui: make action link focus behavior more correcter.
tl;dr - I was totally wrong, <a href> does get focused when the mouse is
pressed, it just inconveniently doesn't show a focus outline in this case.
This CL actually focuses action links on mouse press, but hides the outline if
focus is initiated via mouse press. It also adds behavior to prevent text
selection when the mouse is dragged (to match more odd <a href> behavior).
R=dmazzoni@chromium.org
BUG=441233
Review URL: https://codereview.chromium.org/
805423003
Cr-Commit-Position: refs/heads/master@{#308753}