Moves Blink mixed content enums and helper methods to public.

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 302 matching lines @@
       'exported/WebDragData.cpp',
       'exported/WebEncryptedMediaClient.cpp',
       'exported/WebEncryptedMediaKeyInformation.cpp',
       'exported/WebEncryptedMediaRequest.cpp',
       'exported/WebFederatedCredential.cpp',
       'exported/WebFileSystemCallbacks.cpp',
       'exported/WebFont.cpp',
       'exported/WebFontDescription.cpp',
       'exported/WebHTTPBody.cpp',
       'exported/WebHTTPLoadInfo.cpp',
+      'exported/WebImage.cpp',
       'exported/WebImageGenerator.cpp',
-      'exported/WebImage.cpp',
       'exported/WebMediaConstraints.cpp',
       'exported/WebMediaDeviceInfo.cpp',
       'exported/WebMediaPlayerClient.cpp',
       'exported/WebMediaPlayerEncryptedMediaClient.cpp',
       'exported/WebMediaPlayerSource.cpp',
       'exported/WebMediaRecorderHandler.cpp',
       'exported/WebMediaStream.cpp',
       'exported/WebMediaStreamSource.cpp',
       'exported/WebMediaStreamTrack.cpp',
       'exported/WebMediaStreamTrackSourcesRequest.cpp',
       'exported/WebMessagePortChannelClient.cpp',
+      'exported/WebMixedContent.cpp',
       'exported/WebPasswordCredential.cpp',
       'exported/WebPrerender.cpp',
       'exported/WebPrerenderingSupport.cpp',
       'exported/WebPresentationAvailabilityObserver.cpp',
       'exported/WebPresentationController.cpp',
       'exported/WebRTCAnswerOptions.cpp',
       'exported/WebRTCConfiguration.cpp',
       'exported/WebRTCICECandidate.cpp',
       'exported/WebRTCOfferOptions.cpp',
       'exported/WebRTCSessionDescription.cpp',
@@ skipping 987 matching lines @@
       ['use_default_render_theme==1',
         {
           'platform_test_files': [
             'scroll/ScrollbarThemeAuraTest.cpp',
           ],
         }
       ],
     ],
   },
 }