Added the detailed description for MSVC warning in BUILD.gn

BUG=

Review URL: https://codereview.chromium.org/588603002

Cr-Commit-Position: refs/heads/master@{#296633}

skia/BUILD.gn

@@ -238,15 +238,15 @@ config("skia_library_config") {
     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).
     ]
   }
 }