[NFC][TSan] Merge ScopedReportBase into ScopedReport (#228640)

ScopedReport is the only subclass of ScopedReportBase and nothing uses
ScopedReportBase directly. Merge ScopedReportBase into ScopedReport.

Assisted-by: Gemini
GitOrigin-RevId: e94b7487cf8ebe71744cd4137c9a0968affa0ec2
2 files changed