Bug 1128989 - part 1 - define Telemetry::ID as a uint32_t; r=gfritzsche

We would like to be able to forward-declare Telemetry::ID, so fewer
header files need to include Telemetry.h.  Empirical evidence shows that
forward-declaring enums requires either enum classes, or plain
fixed-width enums.  Let's opt for the latter here; there's no need to
constrain the size to 16 bits, since Telemetry::IDs are rarely stored in
data structures.

toolkit/components/telemetry/gen-histogram-enum.py

@@ -16,7 +16,7 @@ def main(argv):
     filename = argv[0]
 
     print banner
-    print "enum ID {"
+    print "enum ID : uint32_t {"
     for histogram in histogram_tools.from_file(filename):
         cpp_guard = histogram.cpp_guard()
         if cpp_guard: