Make invokeChild just return a string containing the child function call

Also removes several unused chunks of code that were declaring unused
variables, etc.

Change-Id: I47458736b189d59c0448c6f58b60a9b4ab046db2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266565
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
20 files changed