[Extensions] Add missing include

Fix failure from crrev.com/2097de33a1f3d04e78d93e9e2f16aad4b97e47d7

BUG=665517
BUG=660498
NOTRY=true (cast_shell_linux flaking out of control)

Review-Url: https://codereview.chromium.org/2502063002
Cr-Commit-Position: refs/heads/master@{#432274}
1 file changed