The CompilationUnitBuilder of the StreamingProcessor is cleared when an
error occurs in the streaming decoder. The clearing of the
CompilationUnitBuilder was guarded by the existence of the
ModuleCompiler, because this ModuleCompiler and the
CompilationUnitBuilder are created together. However, the
CompilationUnitBuilder is reset when the next section after the code
section is processed, whereas the ModuleCompiler exists until the end of
the AsyncCompileJob. With this CL the clearing of the
CompilationUnitBuilder is also guarded by its own existence.

R=clemensh@chromium.org

Bug: chromium:805346
Change-Id: I0e9e9eaff9239fadb21c0f17990da61cbfaa6856
Reviewed-on: https://chromium-review.googlesource.com/883527
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50844}