Added the detailed description for MSVC warning in BUILD.gn

skia/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
 
@@ skipping 216 matching lines @@
 
   if (is_win) {
     include_dirs = [
       "//third_party/skia/include/utils/win",
       "//third_party/skia/src/utils/win",
     ]
 
     defines += [ "SK_FONTHOST_USES_FONTMGR" ]
 
     cflags = [
-      # TODO(brettw) comment what these are.
-      "/wd4244",
-      "/wd4267",
-      "/wd4341",
-      "/wd4345",
-      "/wd4390",
-      "/wd4554",
-      "/wd4748",
-      "/wd4800",
+      "/wd4244", # conversion from 'type1( __int64)' to 'type2 (unsigned int)'
+      "/wd4267", # conversion from 'size_t' (64 bit) to 'type'(32 bit).
+      "/wd4341", # signed value is out of range for enum constant.
+      "/wd4345", # Object is default-initialized if initialization is omitted.
+      "/wd4390", # ';'empty statement found in looping;is it what was intended?
+      "/wd4554", # 'operator' : check operator precedence for possible error
+      "/wd4748", # compiler will disable optimizations if a function has inline
+                 # assembly code contains flow control(jmp or jcc) statements.
+      "/wd4800", # forcing value to bool 'true/false'(assigning int to bool).
     ]
   }
 }
 
 component("skia") {
   sources = [
     # Chrome sources.
     "config/SkUserConfig.h",
     "ext/analysis_canvas.cc",
     "ext/analysis_canvas.h",
@@ skipping 448 matching lines @@
     ":skia_config",
     "//build/config/compiler:no_chromium_code"
   ]
 
   deps = [
     "//base",
   ]
 
   visibility = [ ":skia" ]
 }