Only set memory tool define if not set.
Only set memory tool define if not set.

This fixes a build issue where MEMORY_TOOL_REPLACES_ALLOCATOR may already be
set.

Review-Url: https://codereview.chromium.org/1951383002

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium