To avoid accessing the heap during asm.js compilation, use the pending
error handler to store the pending warnings such that they can be reported
later during finalization.

As part of this change, refactor PendingCompilationErrorHandler to have a
MessageDetails class holding details of either error or warning messages.

BUG=v8:5203

Change-Id: I5b09254f8899b8dc57d94f1986c7183da847eae3
Reviewed-on: https://chromium-review.googlesource.com/735607
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49007}