Oilpan: Introduce class MainThreadOnly

Oilpan: Introduce class MainThreadOnly

In order to avoid listing up DOM classes used only by the main thread in ThreadState.h,
this CL introduces class MainThreadOnly. If a class X is used only by the main thread,
you can make X inherit from MainThreadOnly.

MainThreadOnly is just a hint for performance optimization.
There is no problem in not specifying MainThreadOnly.
Currently only Node and NodeList inhert from MainThreadOnly.

This CL removes USED_FROM_MULTIPLE_THREADS macros.

BUG=420515

[haraken, 2014-10-16 11:29:00]

haraken@chromium.org changed reviewers:
+ oilpan-reviews@chromium.org, tkent@chromium.org

[haraken, 2014-10-16 11:29:01]

PTAL

[Vyacheslav Egorov (Google), 2014-10-16 11:40:24]

vegorov@google.com changed reviewers:
+ vegorov@google.com

[Vyacheslav Egorov (Google), 2014-10-16 11:40:24]

DBC (historical)

This potentially has issue with circular dependencies.

 If WTF::IsSubclass<T, MainThreadOnly> is instantiated *before* class definition
for T is seen then WTF::IsSubclass<T, MainThreadOnly>::value will just be
`false` silently without any indication/failure. 

Furthermore if after this compiler sees class definition for T saying that T is
subclass of MainThreadOnly it will still evaluate WTF::IsSubclass<T,
MainThreadOnly>::value to false, because that instantiation already happened. 

This will not lead to crashes but this might potentially lead to inconsistencies
in performance. 

This is the reason why we did not use inheritance for this Trait - it simply
does not work as reliably.

[haraken, 2014-10-16 11:55:00]

On 2014/10/16 11:40:24, Vyacheslav Egorov (Google) wrote:
> DBC (historical)
> 
> This potentially has issue with circular dependencies.
> 
>  If WTF::IsSubclass<T, MainThreadOnly> is instantiated *before* class
definition
> for T is seen then WTF::IsSubclass<T, MainThreadOnly>::value will just be
> `false` silently without any indication/failure. 
> 
> Furthermore if after this compiler sees class definition for T saying that T
is
> subclass of MainThreadOnly it will still evaluate WTF::IsSubclass<T,
> MainThreadOnly>::value to false, because that instantiation already happened. 
> 
> This will not lead to crashes but this might potentially lead to
inconsistencies
> in performance. 
> 
> This is the reason why we did not use inheritance for this Trait - it simply
> does not work as reliably.

Thanks for the background! That sounds like a valid reason for not doing this...