Bug 1374499 - Increase number of shmems pre-allocated by PChromiumCDM to 6. r=gerald

Currently we preallocate 4 shmems for ferrying video frames between the GMP and CDM
procecss. Our telemetry indicates that 88% of PChromiumCDM instances need only 4.
So 12% are finding that insufficient, and are taking the slow path and sending frames
back to the content process via a non-shared path while they allocate more shmem.

If we increased our pre-allocation to 6 shmems, we'd guess correctly for 97%, and thus
we'd avoid the slow path more often.


MozReview-Commit-ID: 9HcrlskiCPq

--HG--
extra : rebase_source : 0b3ba593b7b8c9739221c056687f0f3f0fb3019d

modules/libpref/init/all.js

@@ -413,7 +413,7 @@ pref("media.wave.enabled", true);
 pref("media.webm.enabled", true);
 
 pref("media.eme.chromium-api.enabled", true);
-pref("media.eme.chromium-api.video-shmems", 4);
+pref("media.eme.chromium-api.video-shmems", 6);
 
 #ifdef MOZ_APPLEMEDIA
 #ifdef MOZ_WIDGET_UIKIT