Fix sky include guards to match Chromium style

sky/engine/wtf/dtoa/bignum.h

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-#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"
 
 namespace WTF {
 
 namespace double_conversion {
 
     class Bignum {
     public:
         // 3584 = 128 * 28. We can represent 2^3584 > 10^1000 accurately.
@@ skipping 95 matching lines @@
         // The Bignum's value equals value(bigits_) * 2^(exponent_ * kBigitSize).
         int exponent_;
 
         DISALLOW_COPY_AND_ASSIGN(Bignum);
     };
 
 }  // namespace double_conversion
 
 } // namespace WTF
 
-#endif  // DOUBLE_CONVERSION_BIGNUM_H_
+#endif  // SKY_ENGINE_WTF_DTOA_BIGNUM_H_