Update V8 to r6768 as required by WebKit r78450

Change-Id: Ib8868ff7147a76547a8d1d85f257ebe8546a3d3f
diff --git a/src/bignum.cc b/src/bignum.cc
index dd1537a..a973974 100644
--- a/src/bignum.cc
+++ b/src/bignum.cc
@@ -67,7 +67,7 @@
   int needed_bigits = kUInt64Size / kBigitSize + 1;
   EnsureCapacity(needed_bigits);
   for (int i = 0; i < needed_bigits; ++i) {
-    bigits_[i] = value & kBigitMask;
+    bigits_[i] = static_cast<Chunk>(value & kBigitMask);
     value = value >> kBigitSize;
   }
   used_digits_ = needed_bigits;
@@ -266,7 +266,7 @@
   }
   while (carry != 0) {
     EnsureCapacity(used_digits_ + 1);
-    bigits_[used_digits_] = carry & kBigitMask;
+    bigits_[used_digits_] = static_cast<Chunk>(carry & kBigitMask);
     used_digits_++;
     carry >>= kBigitSize;
   }
@@ -287,13 +287,13 @@
     uint64_t product_low = low * bigits_[i];
     uint64_t product_high = high * bigits_[i];
     uint64_t tmp = (carry & kBigitMask) + product_low;
-    bigits_[i] = tmp & kBigitMask;
+    bigits_[i] = static_cast<Chunk>(tmp & kBigitMask);
     carry = (carry >> kBigitSize) + (tmp >> kBigitSize) +
         (product_high << (32 - kBigitSize));
   }
   while (carry != 0) {
     EnsureCapacity(used_digits_ + 1);
-    bigits_[used_digits_] = carry & kBigitMask;
+    bigits_[used_digits_] = static_cast<Chunk>(carry & kBigitMask);
     used_digits_++;
     carry >>= kBigitSize;
   }
@@ -748,7 +748,8 @@
   for (int i = 0; i < other.used_digits_; ++i) {
     DoubleChunk product = static_cast<DoubleChunk>(factor) * other.bigits_[i];
     DoubleChunk remove = borrow + product;
-    Chunk difference = bigits_[i + exponent_diff] - (remove & kBigitMask);
+    Chunk difference =
+        bigits_[i + exponent_diff] - static_cast<Chunk>(remove & kBigitMask);
     bigits_[i + exponent_diff] = difference & kBigitMask;
     borrow = static_cast<Chunk>((difference >> (kChunkSize - 1)) +
                                 (remove >> kBigitSize));