Bug 963498 - Add memory tracking to the Developer HUD. r=vingtetun
bug 952695

* Clear all completed downloads from mozDownloadManager when Gaia Download Manager inits.
* update mockMozDownloadManager to have getDownloads() and clearAllDone() for unit-test purposes.

r=crdlc
Merge pull request #16637 from nullaus/bug952695

bug 952695
Bug 977337 - disabling intermittent test for further investigation
Merge pull request #15870 from jankeromnes/memory

Bug 963498 - Display app memory information. r=vingtetun
Revert "Merge pull request #16656 from Rik/telephony-promise-969280" for breaking gaia-unit test

This reverts commit 52ad4108920c48d68565992a31fc3901ab5168c3, reversing
changes made to 89950a14ec912d78f44345bd143fab1e0da23804.
WIP patch bug 959694