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Previously CdmAdapter only allows one outstanding SendPlatformChallenge() call.
However, CDM actually needs to make parallel SendPlatformChallenge() calls. This
CL fixes CdmAdapter to allow this.

BUG=393722
TEST=Tested on ChromeOS build on Linux with hack in
PepperPlatformVerificationMessageFilter to return faked challenge platform reply.

Review URL: https://codereview.chromium.org/393713003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283651 0039d316-1c4b-4281-b951-d872f2087c98