GC verification is overly weak in Release mode

[lexprfuncall]

The RawObject::Verify checks are embedded in ASSERT statements which mean they are compiled-away in Release mode. Given that the pre- and post-verification are controlled by flags and therefore optional, there seems to be no compelling reason to have the verification do such a more limited consistency check in Release mode. In fact, while valuable, the compiled-away checks have a relatively low runtime cost.

[kasperl]

Added Triaged label.

[iposva-google]

Please work with Siva to beef the validation up.

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Set owner to @lexprfuncall.
Added Accepted label.

[lexprfuncall]

https://chromiumcodereview.appspot.com/10693044

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Added Started label.

[lexprfuncall]

Added Fixed label.

[lexprfuncall]

Three checks are performed in both release and debug builds

1. reserved bits are clear
2. class id tag is a valid index into the class id table
3. size tag agrees with the size computed from the class object