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Added fuzzy padding to the Key value generation, plus included two known
colliding values to be used with approximately 1% chance.

I was trying to discover crbug.com/973943 with this improvement, but have not
succeeded so far. However, the fuzz target seems to be running much faster now
and keeps discovering new coverage starting from the existing corpus. This is
a good indicator that the change makes sense. I've also compared the code
coverage reports locally to make sure those pieces of code under key size checks
are now getting executed, such as:

https://chromium-coverage.appspot.com/reports/706240_fuzzers_only/linux/chromium/src/net/disk_cache/blockfile/entry_impl.cc.html#L449

and

https://chromium-coverage.appspot.com/reports/706240_fuzzers_only/linux/chromium/src/net/disk_cache/blockfile/entry_impl.cc.html#L793

Also fixed a presubmit warning regarding the use of std::to_string.

Bug: 990644, 973943
Change-Id: I0c7560f700ccaf5e25bda2e1d56f3cb613bbe91f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865408Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707063}