Track ascii-ness of data in externalized strings.

Track ascii-ness of data in externalized strings.

If a two-byte string only contains ascii characters, then we can save
memory when flattening a cons string containing it. Similarly we can
use this in Array.prototype.join implementation. To track this a new
bit is added to instance type. This bit is used as a hint in generated
code and in runtime functions.

To enable testing a new V8 extension is added controlled by
--expose-externalize-string flag.

Committed: http://code.google.com/p/v8/source/detail?r=4894

[Søren Thygesen Gjesse, 2010-06-10 13:55:46]

Drive by comment

http://codereview.chromium.org/2762008/diff/3001/4010
File src/objects-inl.h (right):

http://codereview.chromium.org/2762008/diff/3001/4010#newcode250
src/objects-inl.h:250: bool String::HasAsciiChars() {
HasAsciiChars() -> HasOnlyAsciiChars()?

[Mads Ager (chromium), 2010-06-11 06:55:22]

LGTM