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_base_window.h

@@ -9,6 +9,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <string_view>
 #include <vector>
 
 #include "content/public/browser/browser_task_traits.h"
@@ -249,7 +250,7 @@ class BaseWindow : public gin_helper::TrackableObject<BaseWindow>,
   void RemoveFromParentChildWindows();
 
   template <typename... Args>
-  void EmitEventSoon(base::StringPiece eventName) {
+  void EmitEventSoon(std::string_view eventName) {
     content::GetUIThreadTaskRunner({})->PostTask(
         FROM_HERE,
         base::BindOnce(base::IgnoreResult(&BaseWindow::Emit<Args...>),