Warn on ignored return value from CheckedNumeric::AssignIfValid()

Adds a compiler warning that works in all compilation modes on
Clang/GCC, and in Prefast on MSVC. The rationale is all other
value-extracting CheckedNumeric<> functions already prevent the
caller from ignoring the validity state, but without this change
calling AssignIfValid() could silently ignore an invalid state.

Change-Id: I7e16f3cf7f026151ef39215bfe01c05e7c58b32f
Reviewed-on: https://chromium-review.googlesource.com/580296Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Justin Schuh <jschuh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488707}

base/numerics/checked_math.h

@@ -61,7 +61,13 @@ class CheckedNumeric {
   // and is within the range supported by the destination type. Returns true if
   // successful and false otherwise.
   template <typename Dst>
-  constexpr bool AssignIfValid(Dst* result) const {
+#if defined(__clang__) || defined(__GNUC__)
+  __attribute__((warn_unused_result))
+#elif defined(_MSC_VER)
+  _Check_return_
+#endif
+  constexpr bool
+  AssignIfValid(Dst* result) const {
     return IsValid<Dst>() ? ((*result = static_cast<Dst>(state_.value())), true)
                           : false;
   }

content/browser/indexed_db/database_impl.cc

@@ -330,8 +330,7 @@ void DatabaseImpl::Put(
   }
   UMA_HISTOGRAM_COUNTS_1000("WebCore.IndexedDB.PutBlobsCount",
                             blob_info.size());
-  uint64_t blob_size = 0;
-  total_blob_size.AssignIfValid(&blob_size);
+  uint64_t blob_size = total_blob_size.ValueOrDefault(0U);
   if (blob_size != 0) {
     // 1KB to 1GB.
     UMA_HISTOGRAM_COUNTS_1M("WebCore.IndexedDB.PutBlobsTotalSize",