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This method of ELFSymbolizer can be used to wait for the
completion of all async tasks, then allow additional ones
to be performed after that.

This is unlike the Join(), which waits for completion then
terminates all addr2line processes immediately, making any
future call to SymbolizeAsync() crash at runtime.

BUG=755225
R=pasko@chromium.org, lizeb@chromium.org, agrieve@chromium.org,jbudorick@chromium.org

Change-Id: Iea50c5e06bf495390dc1bc50a81151e069841175
Reviewed-on: https://chromium-review.googlesource.com/1046846Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556459}