The String constructor was somewhat complex with a lot of micro
optimizations that are not relevant or even misguided. It would be
really hard to port that code to ES6, which requires String to be
subclassable. So as a first step we reduced the necessary complexity
to the bare minimum (also removing the last user of the fairly complex
MacroAssembler::LookupNumberStringCache method).

This also removes the counters for the String constructor, which
were not properly exposed anymore (and not kept in sync with inlined
versions of the String constructor anyway).

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1335193002

Cr-Commit-Position: refs/heads/master@{#30706}