Add debug logging when symbolizing content_shell.

It seems like we're seeing timeouts when trying to symbolize the
content_shell binary on the Linux ASAN bots. This patch adds some
debug logging to keep buildbot from concluding that the step is hung and
aborting; hopefully this will give us more of a clue as to what's going
on and/or convert an aborted step to a hung step (which should be easier
to debug).

TBR=jochen@chromium.org, glider@chromium.org
BUG=402051

Review URL: https://codereview.chromium.org/563313006

git-svn-id: svn://svn.chromium.org/blink/trunk@182025 bbb929c8-8fbe-4397-9dbb-9b2b20218538