mojo: Only expose std::string_view from StringDataView

Rather than separate storage() and size() accessors, combine them into
a single std::string_view value(). This simplifies the callers which all
use these methods to construct a std::string_view anyways and more
safely encodes the fact that this type does not represent a null-
terminated string.

Change-Id: Ic0e29176d9591bedb73e44a77df35555ca3d78e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6240057
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417164}