Remove incorrect DCHECK in ~BrowserCdmManager.

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
Committed: https://crrev.com/7f59ee147777a9df9e5a83462c6a230ad060e81e
Cr-Commit-Position: refs/heads/master@{#338312}

[davidben, 2015-07-10 16:32:35]

davidben@chromium.org changed reviewers:
+ xhwang@chromium.org

[xhwang, 2015-07-10 17:04:11]

On 2015/07/10 16:32:36, David Benjamin wrote:

Good catch. I missed this.

Yes, now we are controlling tightly the lifetime of
BrowserCdmManagerProcessWatcher. But we can't make assumptions on the lifetime
of BrowserCdmManager. For example, there may be a pending task in the message
loop that holds the last ref count of it.

[xhwang, 2015-07-10 17:04:17]

The CQ bit was checked by xhwang@chromium.org

[xhwang, 2015-07-10 17:04:17]

lgtm

[commit-bot: I haz the power, 2015-07-10 17:04:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1235653002/1

[commit-bot: I haz the power, 2015-07-10 17:22:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-10 17:23:07]

Patchset 1 (id:??) landed as
https://crrev.com/7f59ee147777a9df9e5a83462c6a230ad060e81e
Cr-Commit-Position: refs/heads/master@{#338312}