Add Read/WriteSizeT() functions to the pickle layer, plus one consumer.

base/pickle.cc

Index: base/pickle.cc
diff --git a/base/pickle.cc b/base/pickle.cc
index 2d7a051b20077e20d86346867f414cb3ab961ea6..d461f413dfd2e68a6e97c9f15d5d97dc68faacb3 100644
--- a/base/pickle.cc
+++ b/base/pickle.cc
@@ -106,6 +106,16 @@ bool PickleIterator::ReadUInt64(uint64* result) {
   return ReadBuiltinType(result);
 }
 
+bool PickleIterator::ReadSizeT(size_t* result) {
+  // Always read size_t as a 64-bit value to ensure compatibility between 32-bit
+  // and 64-bit processes.
+  uint64 result_uint64 = 0;
+  bool success = ReadBuiltinType(&result_uint64);
+  *result = static_cast<size_t>(result_uint64);
+  // Fail if the cast above truncates the value.
+  return success && (*result == result_uint64);
+}
+
 bool PickleIterator::ReadFloat(float* result) {
   // crbug.com/315213
   // The source data may not be properly aligned, and unaligned float reads