Add reset and findLastOf to SkSLString.

reset and findLastOf are defined in SkString, but SkSLString may extend
from std::string instead. In that situation, this implements reset and
findLastOf in terms of the std::string API so taht the SkSLString API is
consistent regardless of SKSL_USE_STD_STRING's status.

Bug: skia:
Change-Id: Ifc38a9f3d533d09282bb6243421791dcf783e131
Reviewed-on: https://skia-review.googlesource.com/148382
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
2 files changed