Make InstanceType enum a uint8_t

This enforces that its enumeration values fit in a byte, as required
by Map's {instance_type} field (and probably other parts of the
system).

Clang helpfully emits this error message if an enum value goes out
of range:

   enumerator value 256 is not representable in the underlying type 'uint8_t' (aka 'unsigned char')

Change-Id: I533cd5afc755e7163c2fd40f7b00d9adfd960895
Reviewed-on: https://chromium-review.googlesource.com/565892Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46544}

src/objects.h

@@ -615,7 +615,7 @@ static inline bool IsShortcutCandidate(int type) {
   return ((type & kShortcutTypeMask) == kShortcutTypeTag);
 }
 
-enum InstanceType {
+enum InstanceType : uint8_t {
   // String types.
   INTERNALIZED_STRING_TYPE = kTwoByteStringTag | kSeqStringTag |
                              kInternalizedTag,  // FIRST_PRIMITIVE_TYPE

tools/gen-postmortem-metadata.py

@@ -370,7 +370,7 @@ def load_objects_from_file(objfilename, checktypes):
         # do so without the embedded newlines.
         #
         for line in objfile:
-                if (line.startswith('enum InstanceType {')):
+                if (line.startswith('enum InstanceType : uint8_t {')):
                         in_insttype = True;
                         continue;