Fix :rfc1034identifier when the value contains underscore.
Fix :rfc1034identifier when the value contains underscore.

The :rfc1034identifier substitution converts invalid characters to '-'
because an underscore is invalid in an URL, however, if the value does
contain an underscore (like is the case for target_name in Chromium) it
is not replaced by a '-'.

Fix this by adding an '_' in the IDENT_RE. This cause the '_' to be
replaced by a '-' when using :rfc1034identifier and by a '_' when using
:identifier (i.e. no-operation).

BUG=625875
R=justincohen@chromium.org

Review URL: https://codereview.chromium.org/2141973002 .

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