Fix build break, set initial value for msan variable.

R=zra@google.com

Review URL: https://codereview.chromium.org/2388813003 .

runtime/tools/gyp/runtime-configurations.gypi

@@ -10,6 +10,7 @@
     'dart_io_support%': 0,
     'dart_io_secure_socket%': 1,
     'asan%': 0,
+    'msan%': 0,
     # Intel VTune related variables.
     'dart_vtune_support%': 0,
     'conditions': [

tools/gyp/all.gypi

@@ -14,6 +14,8 @@
     'dart_io_support': 1,
     # Flag that tells us whether this is an ASAN build.
     'asan%': 0,
+    # Flag that tells us whether this is a MSAN build.
+    'msan%': 0,
   },
   'conditions': [
     [ 'OS=="linux"', {