Skip missing translations when generating JSON resources.

It doesn't make sense to replace missing translations with english
strings because Chrome can do this automatically when getting
resources.

BUG=369572

Review-Url: https://codereview.chromium.org/1676793002
Cr-Original-Original-Commit-Position: refs/heads/master@{#480994}
Committed: https://chromium.googlesource.com/chromium/src/+/d2f53be161b01a4cf04dfa387b66bda710f601c0
Review-Url: https://codereview.chromium.org/1676793002
Cr-Original-Commit-Position: refs/heads/master@{#482826}
Committed: https://chromium.googlesource.com/chromium/src/+/1deaebd297b7212932be4afe34ae0b60c45bd9fd
Review-Url: https://codereview.chromium.org/1676793002
Cr-Commit-Position: refs/heads/master@{#483512}
2 files changed