Bug 990353 - Make the decision to discard source entirely per-global, rather than per-script. r=luke

This is effectively a policy decision based on the kind of code we expect to be
running somewhere. This is in contrast to lazy source, which is often a practical
per-script consideration of whether or not we can retrieve the source if requested.

More importantly, tracking this information on the global is much easier to
get right than tracking it on the script.
Bug 990353 - Add a pref for system source discarding. r=bent
Bug 990353 - Flag for discarding where appropriate. r=bent
Bug 990353 - Give tests/chrome its own moz.build file, like tests/mochitest. r=mshal

We also add a mochitest.ini file, which will eventually be used for http-served
files that are associated with chrome mochitests.
Bug 990353 - Tests. r=bent
Bug 990353 - Flip on source discarding for b2g. r=bent
Bug 990353 - Flip off source discarding for XPCShell. r=fabrice