Modernize equality operators in //content

This CL is pure clean-up and contains no functionality changes.
Depending on the files covered in the CL (since the CL was generated
using git cl split), it does a subset of the following:
the following:
- Remove unneeded operator!= declarations/definitions since C++20 can
  automatically derive those from operator==.
- Default operator== where this is equivalent to the current behavior.
- Default operator<=> where this is equivalent to the current
  behavior.

This CL was uploaded by git cl split.

R=haraken@chromium.org

Bug: 40256175
Change-Id: I0c43481411752d21803540aa485c3156cdedb5f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6555890
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461335}

content/renderer/render_frame_impl_browsertest.cc

@@ -550,7 +550,6 @@ struct SourceAnnotation {
     return document_url == rhs.document_url &&
            render_frame_event == rhs.render_frame_event;
   }
-  bool operator!=(const SourceAnnotation& rhs) const { return !(*this == rhs); }
 };
 
 std::ostream& operator<<(std::ostream& out, const SourceAnnotation& s) {