gn: gyp_flag_compare.py de-escape command line properly

Makes some Skia flags be matched properly.

e.g.

  '-DGR_GL_CUSTOM_SETUP_HEADER="GrGLConfig_chrome.h"'
vs
  -DGR_GL_CUSTOM_SETUP_HEADER=\"GrGLConfig_chrome.h\"

Diffs 393202 -> 374424.

It does make the script quite a bit slower unfortunately, I guess the
regular split is native C.

BUG=335824

Review URL: https://codereview.chromium.org/580213003

Cr-Commit-Position: refs/heads/master@{#295562}

tools/gn/bin/gyp_flag_compare.py

@@ -9,6 +9,7 @@ build, report on differences between the command lines."""
 
 
 import os
+import shlex
 import subprocess
 import sys
 
@@ -76,8 +77,7 @@ def GetFlags(lines):
     if 'clang' not in line:
       continue
 
-    # TODO(scottmg): Proper escapes.
-    command_line = line.strip().split()[1:]
+    command_line = shlex.split(line.strip())[1:]
 
     output_name = FindAndRemoveArgWithValue(command_line, '-o')
     dep_name = FindAndRemoveArgWithValue(command_line, '-MF')