Traditionally we add support of a new version of CDM interface in one CL, e.g. [1, 2]. Depending on the scale of the CDM interface change, the CL could be pretty big. 

Also, this requires that we stabilize the CDM interface before Chromium adds supports to it. After we start to ship Chrome versions that support a given CDM interface version, changing the CDM interface could cause incompatibility issues, e.g. a newer CDM used on an older Chrome version. Typically this isn't a big issue, especially since we never ship a real CDM before we finalize the CDM interface and Chromium's support of it, and we bundle the CDM on almost all platforms. But still there's a slight chance of incompatibility issue and if that happens it'll be hard to debug.

To solve these, I propose we add a new flag, e.g. --support-experimental-cdm-interface, and work on the support of a new CDM interface under this flag. Only when we have finalized the CDM interface and Chromium's support, that we enable the support of it by default. Before that, we can use this flag in tests so that we still have proper test coverage.

This approach has the following benefits:
- It allows us to develop the feature gradually, in small CLs, with test coverage.
- Even if we need to change the CDM interface, we can do so without any compatibility issues for normal users.

[1] https://codereview.chromium.org/1023003002
[2] https://codereview.chromium.org/811923002