Clean up DCHECK(==) and .get()s in history backend implementation.
Clean up DCHECK(==) and .get()s in history backend implementation.

Use DCHECK_EQ, DCHECK_LE, DCHECK_GE rather than DCHECK(a==b) etc, to get values printed out when checks fail.

Remove redundant .get()s from is-NULL checks of scoped_ptr<>s.

BUG=524797

Review URL: https://codereview.chromium.org/1314433007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345928}

ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend