Add namespace and-re-arrange PDFium's local copy of chromium /base.

Any projects DEPS'd into chromium and requiring a /base
subset should have a local copy of that subset in a
separate namespace. This will avoid future naming conflicts.

Re-arrange the directory structure to better identify what
came from chromium's base, and to make drop-in replacement
easier for files that contain hard-coded "base/" in their
#include directives.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/900753002