PartitionAlloc: Centralize Partition allocators into one place (Part 1)

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 61 matching lines @@
       'MIMETypeRegistry.cpp',
       'MIMETypeRegistry.h',
       'NotImplemented.cpp',
       'NotImplemented.h',
       'PODArena.h',
       'PODFreeListArena.h',
       'PODInterval.h',
       'PODIntervalTree.h',
       'PODRedBlackTree.h',
       'ParsingUtilities.h',
-      'Partitions.cpp',
-      'Partitions.h',
       'PasteMode.h',
       'PlatformEvent.h',
       'PlatformGestureEvent.h',
       'PlatformInstrumentation.cpp',
       'PlatformInstrumentation.h',
       'PlatformKeyboardEvent.cpp',
       'PlatformKeyboardEvent.h',
       'PlatformMouseEvent.h',
       'PlatformResourceLoader.cpp',
       'PlatformResourceLoader.h',
@@ skipping 907 matching lines @@
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
     'platform_unittest_support_files': [
       'testing/URLTestHelpers.cpp',
       'testing/URLTestHelpers.h',
       'testing/UnitTestHelpers.cpp',
       'testing/UnitTestHelpers.h',
     ],
   },
 }