[Fetch] Use wrapper tracing to express references between wrappers

BodyStreamBuffer has a ReadableStream instance, but because it's
implemented in V8Extra we cannot holds it as a member (i.e., as a
ScriptValue). Instead, we attach the value to the JS wrapper of the
C++ object. This required us to maintain chains of wrappers,
FetchEvent -> Request -> BodyStreamBuffer for example, manually.

This CL replaces that mechanism with wrapper tracing.

Bug: None
Change-Id: I889ef0d0442d62ad50826a4b487ec234f041a982
Reviewed-on: https://chromium-review.googlesource.com/1124270
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573533}
13 files changed