Make String.Utf8 return std::string instead of WTF::CString

As a fourth step to deprecate WTF::CString, this CL makes String::Utf8()
return std::string. So, all uses of Utf8().data() are replaced with
Utf8().c_str() by this CL. Then, all callers of String::Utf8 are
changed to use std::string instead of WTF::CString.

Lastly, if the use of c_str() is unnecessary, this CL removes it
as much as possible.

Bug: 950077
Change-Id: Id475e5fa18e9ea0bc3c7592db21a170f8431dfd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1647060
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#667875}