Stack sampling profiler: increase enable rate to 50%
Stack sampling profiler: increase enable rate to 50%

All crashers of any significance have been addressed as of
85285916c45c2aba7d7b3a0227fd71ec047d758d. This is the next step towards
increasing the enable rate to 100%.

BUG=541419

Review URL: https://codereview.chromium.org/1631953002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#371364}

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