avoid 'may be used uninitialized' warnings in stack_sampling_profiler_unittest.cc

avoid 'may be used uninitialized' warnings in stack_sampling_profiler_unittest.cc

Without this patch, some toolchains may trigger this warning:
../../base/profiler/stack_sampling_profiler_unittest.cc:275:17: error: 'library' may be used uninitialized in this function [-Werror=maybe-uninitialized]

BUG=545051
Committed: https://crrev.com/ad716fc243dac9ea3e950d1cc1adc1abbd71a90d
Cr-Commit-Position: refs/heads/master@{#360647}

[Mostyn Bramley-Moore, 2015-11-19 09:05:04]

The CQ bit was checked by mostynb@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-11-19 09:05:16]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1459033003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1459033003/1

[Mostyn Bramley-Moore, 2015-11-19 09:05:38]

mostynb@opera.com changed reviewers:
+ wittman@chromium.org

[Mostyn Bramley-Moore, 2015-11-19 09:05:38]

@Mike: please take a look at this small followup to
https://codereview.chromium.org/1423583002

[commit-bot: I haz the power, 2015-11-19 10:35:02]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-19 10:35:03]

Dry run: This issue passed the CQ dry run.

[Mike Wittman, 2015-11-19 16:14:50]

lgtm

[Mostyn Bramley-Moore, 2015-11-19 19:45:22]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2015-11-19 19:45:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1459033003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1459033003/1

[commit-bot: I haz the power, 2015-11-19 20:06:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-19 20:08:31]

Patchset 1 (id:??) landed as
https://crrev.com/ad716fc243dac9ea3e950d1cc1adc1abbd71a90d
Cr-Commit-Position: refs/heads/master@{#360647}