[NotLanded] Implement the fetching algorithm of the Web Manifest.

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'BackForwardClientImpl.cpp',
       'BackForwardClientImpl.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
@@ skipping 260 matching lines @@
       'tests/CustomEventTest.cpp',
       'tests/FakeWebPlugin.cpp',
       'tests/FakeWebPlugin.h',
       'tests/FrameLoaderClientImplTest.cpp',
       'tests/FrameTestHelpers.cpp',
       'tests/FrameTestHelpers.h',
       'tests/ImeOnFocusTest.cpp',
       'tests/KeyboardTest.cpp',
       'tests/LinkHighlightTest.cpp',
       'tests/ListenerLeakTest.cpp',
+      'tests/ManifestLoaderTest.cpp',
       'tests/MemoryInfo.cpp',
       'tests/MHTMLTest.cpp',
       'tests/OpenTypeVerticalDataTest.cpp',
       'tests/PageSerializerTest.cpp',
       'tests/PaintAggregatorTest.cpp',
       'tests/PinchViewportTest.cpp',
       'tests/PrerenderingTest.cpp',
       'tests/ProgrammaticScrollTest.cpp',
       'tests/RenderGeometryMapTest.cpp',
       'tests/ScrollingCoordinatorChromiumTest.cpp',
@@ skipping 47 matching lines @@
       ['os_posix==1 and OS!="mac"',
         {
           'web_unittest_files': [
             'tests/LocaleICUTest.cpp',
           ],
         }
       ],
     ],
   },
 }