Disable WPO for mini_installer

We need to do this because when doing a PGO build the /GL flag gets automatically set everywhere, and we're hitting this warning: https://msdn.microsoft.com/en-us/library/b8s7ctwz%28v=vs.90%29.aspx because of the custom memset implementation (https://code.google.com/p/chromium/codesearch#chromium/src/chrome/installer/mini_installer/mini_installer.cc&q=mini_ins&sq=package:chromium&l=862)

TBR=robertshield@chromium.org
BUG=309849

Review URL: https://codereview.chromium.org/1035893002

Cr-Commit-Position: refs/heads/master@{#322377}

chrome/installer/mini_installer.gyp

@@ -134,6 +134,7 @@
                   'BasicRuntimeChecks': '0',
                   'BufferSecurityCheck': 'false',
                   'ExceptionHandling': '0',
+                  'WholeProgramOptimization': 'false',
                 },
                 'VCLinkerTool': {
                   'SubSystem': '2',     # Set /SUBSYSTEM:WINDOWS