Remove incorrect DCHECK in ~BrowserCdmManager.
https://codereview.chromium.org/
1225083002 fixed the core issue but left an
incorrect DCHECK in the code. It is not the case that only a single
BrowserCdmManager per RPH ID may be alive at a time, only that a single one
corresponds to a given live process. (A reference-counted type can never be
assumed to have bounded lifetime for the same reason garbage-collected types
can never be assumed to have bounded lifetime.)
This fixes ChildProcessLauncherBrowserTest.ChildSpawnFail's flakiness on
cast_shell_linux when use_openssl is switched.
BUG=506188,462040
Review URL: https://codereview.chromium.org/
1235653002
Cr-Commit-Position: refs/heads/master@{#338312}