chore: migrate base::StringPiece to std::string_view (#40915)

* 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: 4294483

Xref: https://docs.google.com/document/d/1d4RnD1uAE2t4iANR0nXy82ASIPGsPuw2mpO6v6T7JKs

shell/browser/api/electron_api_protocol.cc

@@ -4,6 +4,7 @@
 
 #include "shell/browser/api/electron_api_protocol.h"
 
+#include <string_view>
 #include <vector>
 
 #include "base/command_line.h"
@@ -193,7 +194,7 @@ const char* const kBuiltinSchemes[] = {
 };
 
 // Convert error code to string.
-constexpr base::StringPiece ErrorCodeToString(ProtocolError error) {
+constexpr std::string_view ErrorCodeToString(ProtocolError error) {
   switch (error) {
     case ProtocolError::kRegistered:
       return "The scheme has been registered";