Remove frame pointer from StackHandler.

Remove frame pointer from StackHandler.

This reduces the size of the StackHandler by yet another word. We no
longer need to keep track of the frame pointer, as the stack walk will
be able to recalculate it.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/991893003

Cr-Commit-Position: refs/heads/master@{#27115}