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1. Gets the register context from signal handler and sets all registers
correctly before using libunwind. This avoids crashes if registers are
used for unwinding frames.
2. Removes the need for unwinding signal handler function since we get
the context of the stopped function directly.

BUG=859260,888434

Change-Id: Ib337f20c19e25e2f82e61b08f8e9ed4ac1195fde
Reviewed-on: https://chromium-review.googlesource.com/c/1307055
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605121}