Fix sky include guards to match Chromium style

sky/engine/wtf/dtoa/bignum.h

Index: sky/engine/wtf/dtoa/bignum.h
diff --git a/sky/engine/wtf/dtoa/bignum.h b/sky/engine/wtf/dtoa/bignum.h
index fa996ce36fb74bbd5b77d3f7f5859fea57b10efe..b6ed62beefe17892cc3ffbb2e74fc94e35de2630 100644
--- a/sky/engine/wtf/dtoa/bignum.h
+++ b/sky/engine/wtf/dtoa/bignum.h
@@ -25,8 +25,8 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#ifndef DOUBLE_CONVERSION_BIGNUM_H_
-#define DOUBLE_CONVERSION_BIGNUM_H_
+#ifndef SKY_ENGINE_WTF_DTOA_BIGNUM_H_
+#define SKY_ENGINE_WTF_DTOA_BIGNUM_H_
 
 #include "sky/engine/wtf/dtoa/utils.h"
 
@@ -142,4 +142,4 @@ namespace double_conversion {
 
 } // namespace WTF
 
-#endif  // DOUBLE_CONVERSION_BIGNUM_H_
+#endif  // SKY_ENGINE_WTF_DTOA_BIGNUM_H_