JSBigString to map via MAP_PRIVATE not MAP_SHARED

Summary:
JSBigString was inadvertently changed to a shared mapping. This means
that any changes to the string will be written back to the file. Ensure
we have a private (COW) mapping.

Reviewed By: kodafb

Differential Revision: D14532757

fbshipit-source-id: 6afb9635493496c90904f1432847c2f0da882c58

ReactCommon/cxxreact/JSBigString.cpp

@@ -52,7 +52,7 @@ JSBigFileString::~JSBigFileString() {
 const char *JSBigFileString::c_str() const {
   if (!m_data) {
     m_data =
-      (const char *) mmap(0, m_size, PROT_READ, MAP_SHARED, m_fd, m_mapOff);
+      (const char *) mmap(0, m_size, PROT_READ, MAP_PRIVATE, m_fd, m_mapOff);
     CHECK(m_data != MAP_FAILED)
       << " fd: " << m_fd
       << " size: " << m_size