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/net/node_stream_loader.cc

@@ -4,6 +4,7 @@
 
 #include "shell/browser/net/node_stream_loader.h"
 
+#include <string_view>
 #include <utility>
 
 #include "mojo/public/cpp/system/string_data_source.h"
@@ -135,8 +136,8 @@ void NodeStreamLoader::ReadMore() {
   is_reading_ = false;
   is_writing_ = true;
   producer_->Write(std::make_unique<mojo::StringDataSource>(
-                       base::StringPiece(node::Buffer::Data(buffer),
-                                         node::Buffer::Length(buffer)),
+                       std::string_view{node::Buffer::Data(buffer),
+                                        node::Buffer::Length(buffer)},
                        mojo::StringDataSource::AsyncWritingMode::
                            STRING_STAYS_VALID_UNTIL_COMPLETION),
                    base::BindOnce(&NodeStreamLoader::DidWrite, weak));