reland: add private non-copying SkReadBuffer::readString()

PS1 is https://skia-review.googlesource.com/c/skia/+/171220
PS2 checks for a bad name.

Bug: oss-fuzz:13706, chromium:941425
Change-Id: Ia11a96a755d8d55327c758ef97a055a472035644
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/200941
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>