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

Review URL: https://codereview.chromium.org/1459033003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360647}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk