The asm.js offset table exists in two forms: Delta-encoded in a byte
array, as generated during asm translation, and decoded, for faster
lookup.
This CL moves the encoded version from the {AsmWasmData} and
{WasmModuleObject} to the {WasmModule}, and stores it off-heap in a C++
array instead of a {ByteArray}.
Also, it moves the decoded version off-heap by storing it in a C++ data
structure that makes lookup easy, instead of encoding it again in
another {ByteArray}.

This change is a nice refactoring in itself, but it also prepares adding
more information to the offset table. For reconstructing the source code
of an asm.js function, we will need to store the start and end offsets
of the whole function as well (see linked bug).

R=jkummerow@chromium.org

Bug: chromium:667678
Change-Id: I79b789c3122dd8ba803cedc6bfdcc3d4b1fa0fd4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2011108
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65900}