Oilpan: restrict implicit RawPtr<U> conversions; U must be T-convertible.
Oilpan: restrict implicit RawPtr<U> conversions; U must be T-convertible.

Make RawPtr<U> only be convertible to a RawPtr<T> iff U is type convertible
to T. This restricts the range of implicit conversions available, avoiding
extts ambiguities. It makes RawPtr symmetric with PassRefPtr in this
regard.

This fixes Oilpan compilation following r189188.

TBR=oilpan-reviews
BUG=450662
NOTRY=true

Review URL: https://codereview.chromium.org/864753005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@189132 bbb929c8-8fbe-4397-9dbb-9b2b20218538

XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,kurli/blink-crosswalk,kurli/blink-crosswalk,kurli/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,kurli/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,kurli/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk