minor skslc bugfixes

This fixes the attributes on sk_VertexID, and a backwards test on
SkSL::String::startsWith and ::endsWith.

Change-Id: Icfddfc8ca95454d8646a1771761685c2525b296e
Reviewed-on: https://skia-review.googlesource.com/17398
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed