GN: support compiler_prefix, use it for ccache on bots.

infra/bots/recipe_modules/flavor/gn_flavor.py

 # Copyright 2016 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 default_flavor
 
 """GN flavor utils, used for building Skia with GN."""
 class GNFlavorUtils(default_flavor.DefaultFlavorUtils):
   def supported(self):
     extra_config = self.m.vars.builder_cfg.get('extra_config', '')
@@ skipping 11 matching lines @@
     extra_config  = self.m.vars.builder_cfg.get('extra_config',  '')
 
     cc, cxx = 'cc', 'c++'
     extra_cflags = []
 
     if compiler == 'Clang':
       cc, cxx = 'clang', 'clang++'
     elif compiler == 'GCC':
       cc, cxx = 'gcc', 'g++'
 
+    compiler_prefix = ""
     ccache = self.m.run.ccache()
     if ccache:
-      cc, cxx = '%s %s' % (ccache, cc), '%s %s' % (ccache, cxx)
+      compiler_prefix = ccache
       if compiler == 'Clang':
         # Stifle "argument unused during compilation: ..." warnings.
         extra_cflags.append('-Qunused-arguments')
 
     if extra_config == 'Fast':
       extra_cflags.extend(['-march=native', '-fomit-frame-pointer', '-O3'])
     if extra_config.startswith('SK'):
       extra_cflags.append('-D' + extra_config)
 
     quote = lambda x: '"%s"' % x
     gn_args = ' '.join('%s=%s' % (k,v) for (k,v) in {
         'cc': quote(cc),
         'cxx': quote(cxx),
+        'compiler_prefix': quote(compiler_prefix),
         'extra_cflags': quote(' '.join(extra_cflags)),
         'is_debug': 'true' if configuration == 'Debug' else 'false',
     }.iteritems())
 
     run = lambda title, cmd: self.m.run(self.m.step, title, cmd=cmd,
                                         cwd=self.m.vars.skia_dir, **kwargs)
 
     run('fetch-gn', [self.m.vars.skia_dir.join('bin', 'fetch-gn')])
     run('gn gen', ['gn', 'gen', self.out_dir, '--args=' + gn_args])
     run('ninja', ['ninja', '-C', self.out_dir])