Removes the --ignition flag which is now on by default. Adds a
--stress-fullcodegen flag which enables running all functions supported
by fullcodegen to be compiled by fullcodegen.

This will enable moving parser internalization later when we are not
stressing fullcodegen or compiling asm.js functions.

BUG=v8:5203, v8:6409, v8:6589

Change-Id: I7fa68016d4e734755434ec0b4e749ef65ffa7f4e
Reviewed-on: https://chromium-review.googlesource.com/565569
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46635}