Bug 986544 - Don't try to parse searchplugins list as XML. r=rnewman

XHR requests default to text/xml.  This just avoids trying to interpret
list.txt as XML.
Bug 1083279 - Part 1: Don't get_binary_path on mobile/android. r=gbrown

The logic was cribbed from the final output of |mach run|.
Bug 1083279 - Part 2: Allow SCRIPT_DIRECTORY to depend on the invocation script name. r=ted

This issue manifests itself as the reftest extension not being installed
for local testers.  This is because the extension's location is
determined relative to SCRIPT_DIRECTORY, and for local testers, __file__
ignores the symlink from $OBJDIR/_tests/reftest/runreftest.py ->
$SRCDIR/layout/tools/reftest/runreftest.py.

This was changed in Bug 915865.