Add missing ostream include.

This file has a transitive dependency on ostream via
base/numerics/safe_conversions.h. The ostream include in
safe_conversions.h is however unused and should be removed.

The <ostream> include is needed by the `CHECK*(...) << "..."`
statements. base/check.h only forward declares <ostream> by
including <iosfwd>. The documentation in base/check.h mentions:

// Additional information can be streamed to these macros and will be included
// in the log output if the condition doesn't hold (you may need to include
// <ostream>):
//
//   CHECK(condition) << "Additional info.";

Change-Id: Id5f31c2ca010936bbae2aa512527940247cc787c
Bug: 1270812, 1372522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4152491
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1091466}