Fix build of mini_installer with VS2012

BUG=143646
TEST= mini_installer (with branding=chrome) builds

Here is the error:

t0\src>ninja -C out\Debug mini_installer
[1/9] CXX obj\chrome_frame\npchrome_frame.chrome_tab.obj
FAILED: ninja -t msvc -r . -o obj\chrome_frame\npchrome_frame.chrome_tab.obj -e environment.x86 -- cl.exe ...
 .. microsoft visual studio 11.0\vc\atlmfc\include\atlbase.h(2845) : error C2338: 'CAtlDllModuleT<T>'
 must be used with either _WINDLL or _USRDLL defined
 ..\microsoft visual studio 11.0\vc\atlmfc\include\atlbase.h(3298) : 
see reference to class template instantiation 'ATL::CAtlValidateModuleConfiguration<isDllModule,T>' being compiled
        with
        [
            isDllModule=true,
            T=`anonymous-namespace'::ChromeTabModule
        ]
        t0\src\chrome_frame\chrome_tab.cc(98) : see reference to class template instantiation 'ATL::CAtlDllModuleT<T>' being compiled
        with
        [
            T=`anonymous-namespace'::ChromeTabModule
        ]
Review URL: https://codereview.chromium.org/11189018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@162291 0039d316-1c4b-4281-b951-d872f2087c98

chrome_frame/chrome_frame.gyp

@@ -862,6 +862,9 @@
         '../chrome/chrome.gyp:chrome_version_resources',
         '../chrome/chrome.gyp:common',
       ],
+      'defines': [
+        '_WINDLL',
+      ],
       'sources': [
         'chrome_frame_elevation.rgs',
         'chrome_frame_reporting.cc',