tools/gn: disallow non-canonical integer literals

tools/gn: disallow non-canonical integer literals

In most languages familiar to Chromium developers (e.g., C++,
JavaScript, Python, Java, Go, Bash, Perl, Ruby), a leading 0 in an
integer literal indicates an octal number.  For example, "010" and
"-010" represent 8 and -8, respectively.

GN doesn't have any use for octal integer literals, so reject integers
with leading zeros to avoid confusion rather than interpreting them as
decimal values.

While here, "-0" isn't useful either since it's equivalent to "0", so
reject that too.

No Chromium BUILD.gn files are affected by this change.

Review URL: https://codereview.chromium.org/1038233002

Cr-Commit-Position: refs/heads/master@{#323142}