[build] Explicitly import jumbo.gni

This adds a missing explicit include of jumbo.gni. It is included
indirectly through V8, which blocks removing jumbo support on
the V8 side: https://crrev.com/c/1890090

Bug: 994387
Change-Id: Ib5d3310d57c9bc50c11d4a4339e062dba298e5c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890311Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710747}

weblayer/shell/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
+import("//build/config/jumbo.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/config/win/console_app.gni")