Tidy MethodProtoHelper.

Move to place of only use, class_linker.cc. Be lazy in computing the name.

Before:
WaitTime: 2699
WaitTime: 2791
WaitTime: 2653
WaitTime: 2929
WaitTime: 2651
WaitTime: 2971

After:
WaitTime: 2749
WaitTime: 2786
WaitTime: 2852
WaitTime: 2856
WaitTime: 2703
WaitTime: 2784

Bug: 18054905
Bug: 16828525

(cherry picked from commit 03b6eafb)

Change-Id: I1438efbda58369ddd0ac36eda8a5a0a6c6fdff77