Fix some clang warnings with -Wmissing-braces in pdfium.

Clang warns if there are missing braces around a subobject
initializer. The most common idiom that triggers this is:
  STRUCT s = {0};
if the first field of STRUCT is itself a struct. This can
be more simply written as:
  STRUCT s = {};
which also prevents the warning from firing.

Other instances of the warning have been fixed by adding
braces where appropriate.

R=brucedawson@chromium.org

Review URL: https://codereview.chromium.org/1213523004.
7 files changed