libdebuggerd: store process uptime.

Application developers would like to know how long their process has
been alive for to distinguish between crashes that happen immediately
upon startup and crashes in regular operation.

Test: manual
Change-Id: Ia31eeadfcced358b478c7a7c7bb2e8a0252e30f4
diff --git a/debuggerd/proto/tombstone.proto b/debuggerd/proto/tombstone.proto
index 2c7156b..57a3b3e 100644
--- a/debuggerd/proto/tombstone.proto
+++ b/debuggerd/proto/tombstone.proto
@@ -19,6 +19,9 @@
 
   string process_name = 9;
 
+  // Process uptime in seconds.
+  uint32 process_uptime = 20;
+
   Signal signal_info = 10;
   string abort_message = 14;
   Cause cause = 15;
@@ -28,7 +31,7 @@
   repeated LogBuffer log_buffers = 18;
   repeated FD open_fds = 19;
 
-  reserved 20 to 999;
+  reserved 21 to 999;
 }
 
 enum Architecture {