Make a static const variable constexpr in BitSetIterator

This was found during some changes in Google3.

Also renames BitsPerWord to kBitsPerWord.

Bug:
Change-Id: I80cfe3a391963c2da376a7d8eadfc2797df01894
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19286
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/src/common/BitSetIterator.h b/src/common/BitSetIterator.h
index 432433a..1a7fd60 100644
--- a/src/common/BitSetIterator.h
+++ b/src/common/BitSetIterator.h
@@ -51,7 +51,7 @@
       private:
         unsigned long getNextBit();
 
-        static const size_t BitsPerWord = sizeof(uint32_t) * 8;
+        static constexpr size_t kBitsPerWord = sizeof(uint32_t) * 8;
         std::bitset<N> mBits;
         unsigned long mCurrentBit;
         unsigned long mOffset;
@@ -88,7 +88,7 @@
     if (bits.any()) {
         mCurrentBit = getNextBit();
     } else {
-        mOffset = static_cast<unsigned long>(roundUp(N, BitsPerWord));
+        mOffset = static_cast<unsigned long>(roundUp(N, kBitsPerWord));
     }
 }
 
@@ -120,8 +120,8 @@
             return ScanForward(wordBits) + mOffset;
         }
 
-        mBits >>= BitsPerWord;
-        mOffset += BitsPerWord;
+        mBits >>= kBitsPerWord;
+        mOffset += kBitsPerWord;
     }
     return 0;
 }