Fixing remaining VC++ 2015 64-bit build breaks

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 67fc8e5b554a3bad4a41115cada9f3a0f95b4921..1ae74706030ccc8da9ed448755f6f01ae21da36a 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -3134,6 +3134,7 @@
               '_SCL_SECURE_NO_DEPRECATE',
             ],
             'msvs_disabled_warnings': [
+              # forcing value to bool 'true' or 'false' (performance warning)
               4800,
             ],
             'msvs_settings': {
@@ -5614,7 +5615,7 @@
             # it's enabled. This will generally only be true for system-level
             # installed Express users.
             'msvs_disabled_warnings': [
-              4702,
+              4702, # unreachable code
             ],
           }],
         ],
@@ -5698,6 +5699,14 @@
           # should work through these at some point -- they may be removed from
           # the RTM release in the /W4 set.
           4456, 4457, 4458, 4459,
+
+          # The x64 builds with VS 2015 trigger hundreds of warnings about
+          # pointer truncation but these are all or virtually all by design, and
+          # therefore not worth fixing individually. The C4311 warnings are
+          # particularly common in libxml. The C4312 warnings are particularly
+          # common in libxml and v8.
+          4311, # type cast: pointer truncation from pointer to smaller integer
+          4312, # type cast: conversion from integer to larger pointer
         ],
         'msvs_settings': {
           'VCCLCompilerTool': {