Order is now:
 - forward declarations
 - enums
 - nested classes
 - static consts
 - constructors
 - static methods
 - regular methods
 - members fields

Bonus:
 - Allocate*() methods are grouped together
 - Sections

BUG=

Review URL: https://codereview.chromium.org/1312503004

Cr-Commit-Position: refs/heads/master@{#30327}