Since it doesn't allocate, make UncompiledData::Init a member function,
consistent with SharedFunctionInfo::Init.

Bug: chromium:1011762
Change-Id: I984adf9004193eb9da504ddd39dd95345eccaf82
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1926031
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65076}