Fix UBSAN signed integer overflow

This commit fixes UBSAN signed integer overflow error
in flat_hash_map_unittest.cc.

Fixed error example:
flat_hash_map_unittest.cc:370:11: runtime error:
signed integer overflow: 1083454666 * 2 cannot be represented
in type 'int'.

By default the perfetto_unittests ignores this error and other UBSAN errors.
To detect this issue command `export UBSAN_OPTIONS=log_path="$HOME/ubsan.log"`
can be used before run `perfetto_unittests`.

Change-Id: I18bd939a2eb4c048783d9d625fce45e011df2ea6