This significantly reduces the build time when modifying wasm
files: before touching all wasm headers required 684 steps to
rebuild, now it's 216.

BUG=v8:7754,v8:7490

Change-Id: Id7ff6f9063168556daad4840ee614cf68144cdb2
Reviewed-on: https://chromium-review.googlesource.com/1145264
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54681}