Add RandomString() and its test, and use it everywhere it makes sense

Add RandomString() and its test, and use it everywhere it makes sense

This unifies several things that used a 16-character random string, and
a few other users of random identifiers where it also made sense to use
a 16-character random string.

TEST=crashpad_util_test RandomString.RandomString
R=scottmg@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/4f09b58d1fbd4be3eb8cef2d1682c37854f90df6

[Mark Mentovai, 2015-11-16 17:32:12]

mark@chromium.org changed reviewers:
+ scottmg@chromium.org

[scottmg, 2015-11-16 18:24:32]

Good stuff, lgtm.

https://codereview.chromium.org/1451793002/diff/1/util/misc/random_string_tes...
File util/misc/random_string_test.cc (right):

https://codereview.chromium.org/1451793002/diff/1/util/misc/random_string_tes...
util/misc/random_string_test.cc:60: // extremely unlikley, for a character to
not appear at all.
unlikely

[Mark Mentovai, 2015-11-16 18:39:08]

Committed patchset #2 (id:20001) manually as
4f09b58d1fbd4be3eb8cef2d1682c37854f90df6 (presubmit successful).