Refactor KeySystems code to call a function to populate the info. (relanding r221932)

Refactor KeySystems code to call a function to populate the info. (re-land r221932)

This allows KeySystems to call GetContentClient()->AddKeySystems() so we can
move key system information to chrome/. The new ContentClient API is defined and
called, but we still rely on key_systems_info.cc.

BUG=224793
TEST=Existing content_unittests and content_browsertests pass.
TBR=jam@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=222074

[ddorwin, 2013-09-09 18:01:21]

xhwang, PTAL at just the differences in PS2:
 * PS1 is just the cherry-pick.
 * PS2 fixes the compile error that caused r221932 to be reverted in r221939.

TBRing jam@ since there are no changes to content/ or chrome/ from the original
CL.

The compile error was:
../../content/renderer/media/crypto/key_systems_info.cc: In function ‘void
content::AddPepperBasedWidevine(std::vector<content::KeySystemInfo>*)’:
../../content/renderer/media/crypto/key_systems_info.cc:104:41: error: invalid
conversion from ‘int’ to ‘content::SupportedCodecs’ [-fpermissive]

[xhwang, 2013-09-09 18:03:24]

lgtm

[ddorwin, 2013-09-09 19:50:16]

Committed patchset #2 manually as r222074 (presubmit successful).