Use size_t to fix the build on mac.

Change-Id: Id145bdb711eea9ee33bcd9f258ff3befd6e7e86b
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2465487
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

src/common/dwarf/dwarf2reader.h

@@ -293,7 +293,7 @@ class RangeListReader {
   // Convenience functions to handle the mechanics of reading entries in the
   // ranges section.
   uint64_t ReadULEB(uint64_t offset, uint64_t* value) {
-    uint64_t len;
+    size_t len;
     *value = reader_->ReadUnsignedLEB128(cu_info_->buffer_ + offset, &len);
     return len;
   }