Have uses of MockImageResourceClient leak less.

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 3765 matching lines @@
             'editing/SurroundingTextTest.cpp',
             'editing/VisiblePositionTest.cpp',
             'editing/VisibleSelectionTest.cpp',
             'editing/VisibleUnitsTest.cpp',
             'editing/serializers/StyledMarkupSerializerTest.cpp',
             'events/EventPathTest.cpp',
             'fetch/CachingCorrectnessTest.cpp',
             'fetch/ClientHintsPreferencesTest.cpp',
             'fetch/ImageResourceTest.cpp',
             'fetch/MemoryCacheTest.cpp',
+            'fetch/MockImageResourceClient.cpp',
             'fetch/RawResourceTest.cpp',
             'fetch/ResourceFetcherTest.cpp',
             'fetch/ResourceLoaderOptionsTest.cpp',
             'fetch/ResourceTest.cpp',
             'fileapi/FileListTest.cpp',
             'fileapi/FileTest.cpp',
             'frame/ImageBitmapTest.cpp',
             'frame/OriginsUsingFeaturesTest.cpp',
             'frame/RootFrameViewportTest.cpp',
             'frame/SubresourceIntegrityTest.cpp',
@@ skipping 57 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }