In a few places in the parser base, we were forced to initialize
variables of type ExpressionT even if it's not necessary, as they
are assigned to later on before their use.  This was required
because, for the case of the preparser, ExpressionT is
PreParserExpression which had no default constructor.

This patch adds a default constructor, equivalent to EmptyExpression
for this class, and gets rid of the unnecessary initializations.

R=adamk@chromium.org, littledan@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2162763003
Cr-Commit-Position: refs/heads/master@{#37884}