Currently the output_json method generates revisions for projects that
are in the revision_mapping and only those projects appear in the
manifest and source_manifest, using the generated revision. This method
is used to construct the default json output for bot_update in tests.
This has 2 problems:
1. If a project has a fixed revision, its manifest entries and the
property used the generated revision, meaning the gitiles input revision
will not be used in the json output.
2. If a project has a fixed revision and isn't present in
revision_mapping then manifest entries aren't generated for it.

This changes the method to use the revisions in the fixed_revisions dict
and to create manifest entries for the projects that appear in the
fixed_revisions dict which should make fewer situations where
bot_update's output_json needs to be explicitly set.

Change-Id: I99de52edc9248c28e9d3aa3a04cc77bf9fccbe37
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: chrome_release
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2304644Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>