This CL changes the DECIMAL_LITERAL lexer rule to allow
negative decimal literals as well.

This could also be achieved by using the unary minus operation,
which would occur an runtime overhead and feel counter-intuitive
for literals (imho).

R=tebbi@chromium.org

Change-Id: Ib01aa1930254bcd85a161de385b0fd4f176feb46
Reviewed-on: https://chromium-review.googlesource.com/1027473Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#52773}