Adds a bytecode_age field to BytecodeArray objects. This is incremented each
time the bytecode array is marked by GC, and reset to zero if the bytecode
is executed.

This is used to enable the CompilationCache for interpreted functions,
where Interpreted entries are evicted once the bytecode becomes old.

BUG=chromium:666275,v8:4680

Review-Url: https://codereview.chromium.org/2534763003
Cr-Commit-Position: refs/heads/master@{#41356}