Rename DecimalPrivate namepsace to decimal_private

https://google.github.io/styleguide/cppguide.html#Namespace_Names
> Namespace names are all lower-case.

This CL has no behavior changes.

Bug: 889726
Change-Id: I6e561ba5f0e92758fb2922bd8e41c4feb70f2203
Reviewed-on: https://chromium-review.googlesource.com/c/1301696Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#603167}

third_party/blink/renderer/platform/wtf/decimal.cc

@@ -40,7 +40,7 @@
 
 namespace blink {
 
-namespace DecimalPrivate {
+namespace decimal_private {
 
 static int const kExponentMax = 1023;
 static int const kExponentMin = -1023;
@@ -229,9 +229,9 @@ static uint64_t ScaleUp(uint64_t x, int n) {
   }
 }
 
-}  // namespace DecimalPrivate
+}  // namespace decimal_private
 
-using namespace DecimalPrivate;
+using namespace decimal_private;
 
 Decimal::EncodedData::EncodedData(Sign sign, FormatClass format_class)
     : coefficient_(0), exponent_(0), format_class_(format_class), sign_(sign) {}

third_party/blink/renderer/platform/wtf/decimal.h

@@ -39,7 +39,7 @@
 
 namespace blink {
 
-namespace DecimalPrivate {
+namespace decimal_private {
 class SpecialValueHandler;
 }
 
@@ -63,7 +63,7 @@ class WTF_EXPORT Decimal {
     DISALLOW_NEW();
     // For accessing FormatClass.
     friend class Decimal;
-    friend class DecimalPrivate::SpecialValueHandler;
+    friend class decimal_private::SpecialValueHandler;
 
    public:
     EncodedData(Sign, int exponent, uint64_t coefficient);