Use Stub CDM for building libwidevinecdmadapter.so

- This is a temporary fix for unblocking LLD.
- We use the stub CDM to build the CDM adapter.
- We ship the copied (real) CDM and the CDM adapter.
- Even though the CDM adapter is built against the stub cdm at
  "stub_cdm/libwidevinecdm.so", only the filename is baked
  into the adapter because of the gn dependency,
  e.g., readelf -d libwidevinecdmadapter.so :
  0x0000000000000001 (NEEDED)    Shared library: [libwidevinecdm.so]
- At run time, when loading the CDM adapter, the system will search
  for the CDM in the same dir where the adapter is located. Hence
  the real CDM will be loaded.

BUG=707488

Review-Url: https://codereview.chromium.org/2792743002
Cr-Commit-Position: refs/heads/master@{#462104}
1 file changed