Simplify string handling for invokeChild.

GrFragmentProcessor::invokeChild now takes a string_view for the coords
instead of a String. This reduces copying while still allowing a String
to be passed in without casts/conversions.

Change-Id: I311eedda0ae2de8258c7b3a28420c152626401b2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503337
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed