This allows the main ExpressionCoverGrammar parsing to be a little tighter.

Change-Id: I45e3d1a9a647a98ffe1ad9969cb1ffbe47f67f1c
Reviewed-on: https://chromium-review.googlesource.com/c/1326468
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57362}