Adds a build-time flag to control enabling of V8 Lite mode. Currently
this mode enables optimize-for-size and makes that flag read-only so that
it can't be changed at runtime.

This mode also replaces the --minimal flag which was previously used
to make porting easier.

BUG=v8:8293

Change-Id: I8360b4d55dd15a2a7c18429c94329dc5264dea86
Reviewed-on: https://chromium-review.googlesource.com/c/1276467
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56653}