spanification: CachedMetadata

Replace the Data()/size() pair with a new Data() that returns the
corresponding span. Drop RawData() and RawSize() in favor of the span
accessors. Add a new GetHeader() to capture the intricacies of getting
the header block in a single place.

Rewrite V8LocalCompileHintsConsumer to use a base::SpanReader.

Bug: 351564777
Change-Id: I3325513408b574275eb7937be8198e5bfd37d142
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5921438
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1368697}