Convert ViewMsg_PluginActionAt to MainFrame mojo method

As part of this migration, it was realized that
blink::PluginAction::enable was not used, leaving only
blink::PluginAction::type (blink::PluginActionType) left.

Hence, blink::PluginActionType was replaced by
blink::mojom::PluginActionType enum and call sites adapted.

BUG=1051162, 919392

R=dtapuska@chromium.org
TBR=wjmaclean@chromium.org

Change-Id: I176a2f45b5865755f7b4cf08d5740581eba56793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066877
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#743676}
17 files changed