Added dependency to jinja2 sources for mojom generator

mojom generator uses jinja2 template.
If we update jinja2 library, jinja2 may update api/internal structure.
In such case, all mojom generator using jinja2 may need to re-run.

I added jinja2_sources for such case and use it as inputs dependency for mojom generator action.
It is recommended by gn.
https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#target-declarations-action_declare-a-target-that-runs-a-script-a-single-time-inputs

* Minor fix
Update README.chromium to pass current presubmit check.

Change-Id: I8b7f126c3a807ac211db01c90437d584c9c39f85
Reviewed-on: https://chromium-review.googlesource.com/961763
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#543073}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 18c898ac14ab3b736ec3abca95dccff782f1c351
4 files changed