* chore: migrate from base::StringPiece to std::string_view in keyboard_util.cc
* chore: migrate from base::StringPiece to std::string_view in error_thrower.cc
* chore: migrate from base::StringPiece to std::string_view in electron_api_web_contents.cc
* chore: migrate from base::StringPiece to std::string_view in gin_helper/dictionary.h
* chore: migrate from base::StringPiece to std::string_view in electron_api_url_loader.cc
* chore: phase out internal use of base:::StringPiece
`base::StringPiece` is being phased out upstream. Its code has been
removed upstream and it's just a typedef for `std::string_view`.
They haven't removed the typedef yet, so this PR tries to get ahead
of future breakage by migrating "internal" use (i.e. leaving alone the
places where the `base::StringPiece` name is coming from an upstream
method that we override).
Xref: https://bugs.chromium.org/p/chromium/issues/detail?id=691162
Xref: https://chromium-review.googlesource.com/c/chromium/src/+/4294483
Xref: https://docs.google.com/document/d/1d4RnD1uAE2t4iANR0nXy82ASIPGsPuw2mpO6v6T7JKs
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