Add a unit test for timers

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 917 matching lines @@
       'LayoutUnitTest.cpp',
       'LifecycleContextTest.cpp',
       'PODArenaTest.cpp',
       'PODFreeListArenaTest.cpp',
       'PODIntervalTreeTest.cpp',
       'PODRedBlackTreeTest.cpp',
       'PurgeableVectorTest.cpp',
       'SharedBufferTest.cpp',
       'TestingPlatformSupport.cpp',
       'TestingPlatformSupport.h',
+      'TimerTest.cpp',
       'TracedValueTest.cpp',
       'WebScreenInfoTest.cpp',
       'WebVectorTest.cpp',
       'animation/TimingFunctionTest.cpp',
       'animation/UnitBezierTest.cpp',
       'clipboard/ClipboardUtilitiesTest.cpp',
       'fonts/FontCacheTest.cpp',
       'fonts/FontDescriptionTest.cpp',
       'fonts/FontTest.cpp',
       'fonts/GlyphBufferTest.cpp',
@@ skipping 61 matching lines @@
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
     'platform_unittest_support_files': [
       'testing/URLTestHelpers.cpp',
       'testing/URLTestHelpers.h',
       'testing/UnitTestHelpers.cpp',
       'testing/UnitTestHelpers.h',
     ],
   },
 }