While parsing top-level code eager functions are skipped just like lazy
ones, but also a parse task is created for each.

The parse tasks are run by the compiler dispatcher and can be executed
either on background thread or in idle time.
After parsing of top-level code finishes it waits for all unfinished
parser tasks - possibly picking up and executing them on current thread.
Afterwards parse task results are stitched together with top-level AST,
in case of failures eager functions are treated just like lazy -
parsing/compilation is retriggered for them in the runtime and proper
errors are generated (performance is not optimized for error case at
all).

BUG=v8:6093

Change-Id: I718dd2acc8a70ae1b09c2dea2616716605d7b05d
Reviewed-on: https://chromium-review.googlesource.com/483439
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44849}