[jumbo] Avoid using kSymbolKey name in the blink scope

There is a kSymbolKey in the public API and due to the new
enum shadow warnings, jumbo builds might combine the code in
such an order that the public blink::WebInputEvent::kSymbolKey
shadows the blink::{anonymous namespace}::kSymbolKey.

Note, there is no bug, just an unfortunate false warning and one
that only appears in rare jumbo builds.

Change-Id: Id575b2aef3b3f1be46d3722ed79454212bcf650e
Reviewed-on: https://chromium-review.googlesource.com/c/1333649
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607606}
1 file changed