[mojo][bindings] Omit unnecessary includes in *.mojom-*-forward.h

This change adds conditions to include interface- and struct-related headers only if
the module defines interfaces and structs.

Bug: 959279
Change-Id: I0b47ca088d7894b675f8aa768e1fa505aaae6dce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594060
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#657102}
1 file changed