third_party/WebKit/Source/wtf/ThreadingPrimitives.h got rewritten as follows:

    class WTF_EXPORT MutexBase {
        WTF_MAKE_NONCOPYABLE(MutexBase); USING_FAST_MALLOC(MutexBase);
    public:
        ~MutexBase();
    
        void lock();
        void unlock();
    #if ENABLE(ASSERT)
        bool Locked() { return mutex_.recursion_count_ > 0; }
    #endif

Note that |locked| got rewritten to |Locked|, but |lock| and |unlocked| where not rewritten here.  This is a problem, because |lock| *did* get rewritten in third_party/WebKit/Source/wtf/Locker.h:

    template <typename T> class Locker final {
        STACK_ALLOCATED();
        WTF_MAKE_NONCOPYABLE(Locker);
    public:
        Locker(T& lockable) : lockable_(lockable) { lockable_.Lock(); }
        ~Locker() { lockable_.Unlock(); }
    private:
        T& lockable_;
    };

Note that the repro above happens only after the following 3 CLs are applied:
- https://codereview.chromium.org/2246263002
- https://codereview.chromium.org/2274653002
- https://codereview.chromium.org/2256913002
Without the CLs above, |lock| inside Locker.h doesn't get rewritten (and so inconsistent rewriting in ThreadingPrimitives.h is not a compile problem).  I am guessing that |lock| inside Locker.h is a CXXDependentScopeMemberExpr (which starts getting rewritten in https://codereview.chromium.org/2256913002).