[rpcexplorer] Add support for field masks
[rpcexplorer] Add support for field masks

Field masks are JSON-encoded as a single string, not a message.
Take that into account in code completer.

This CL does not update tests because rpcexplorer development is currently
completely broken: crbug.com/848368

Bug: 848371
Change-Id: Ia874144930324c5fd9dcf7f7d3b2e5f62b2e4af5
Reviewed-on: https://chromium-review.googlesource.com/1082653
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go