[cr webui] no selection of icon buttons

This CL sets user-select: none; on icon buttons. There doesn't appear to
be a need/reason to do selection of a icon button (when drag selecting
for example). This change is primarily to make the context menu (right-
click menu) show options for a button rather than selected text.

Bug: 769209
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I64013d3f087ff6cd3c33b513acc2c2d340406202
Reviewed-on: https://chromium-review.googlesource.com/688499Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504846}

ui/webui/resources/cr_elements/cr_icons_css.html

@@ -17,6 +17,7 @@
         background-size: var(--cr-icon-size);
         flex-shrink: 0;
         height: var(--cr-icon-ripple-size);
+        user-select: none;
         width: var(--cr-icon-ripple-size);
       }