At this time the following files are moved to platform/wtf/:

    Allocator.h
    WTF.h (note: WTF.cpp remains in wtf/)
    allocator/PartitionAllocator.{h,cpp}
    allocator/Partitions.{h,cpp}

WTF.cpp is not moved in this change, but this seems OK since "platform_wtf"
target is defined as source_set. WTF.cpp has heavy dependencies and it's
hard to move WTF.cpp now.

BUG=691465

Review-Url: https://codereview.chromium.org/2762943002
Cr-Commit-Position: refs/heads/master@{#458361}