perf: remove unnecessary `.c_str()` calls (#41869)

* perf: remove unnecessary c_str() call when invoking promise.RejectWithErrorMessage()

RejectWithErrorMessage() takes a std::string_view

* perf: remove unnecessary c_str() call when invoking Environment::SetVar()

the val arg to Environment::SetVar() takes a const std::string&

* refactor: use string_view variant of base::UTF8ToWide()

* perf: remove unnecessary c_str() call when instantiating a ScopedHString

ScopedHString has always taken a StringPiece

* refactor: use simpler invocation of base::make_span()

* perf: remove unnecessary c_str() call when calling base::CommandLine::HasSwitch()

HasSwitch() already takes a string_piece

* perf: remove unnecessary c_str() call when calling net::HttpResponseHeaders::AddHeader()

AddHeader() already takes a StringPiece arg

* perf: omit unnecessary str -> wstr -> str conversion in DesktopCapturer::UpdateSourcesList()

this conversion was made redundant by c670e38b