Revert "Parser & internalization fix: ensure no heap allocs during GetString(Handle<String>)."

This reverts r25155.

Reason: breaks webkit unit tests.

BUG=
TBR=marja@chromium.org

Review URL: https://codereview.chromium.org/707483002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25159 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Make slow tests on arm64

TBR=bmeurer@chromium.org, ishell@chromium.org

Review URL: https://codereview.chromium.org/701023003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25160 ce2b1a6d-e550-0410-aec6-3dcde31c8c00