Avoid using "using" in global scope

"using namespace" in global scope introduces a lot of symbols into the
current scope and does not work well with jumbo. In case of WTF you
can also equally well just write a WTF:: prefix.

Bug: 
Change-Id: Id615f60151e0bc46cfe2a5e9a6a9313c412f1344
Reviewed-on: https://chromium-review.googlesource.com/570446
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487076}

third_party/WebKit/Source/platform/fonts/FontCache.cpp

@@ -62,8 +62,6 @@
 #include "public/platform/Platform.h"
 #include "ui/gfx/font_list.h"
 
-using namespace WTF;
-
 namespace blink {
 
 #if !defined(OS_WIN) && !defined(OS_LINUX)
@@ -413,11 +411,11 @@ void FontCache::CrashWithFontInfo(const FontDescription* font_description) {
   }
 
   FontDescription font_description_copy = *font_description;
-  debug::Alias(&font_description_copy);
+  WTF::debug::Alias(&font_description_copy);
 
-  debug::Alias(&font_cache);
-  debug::Alias(&font_mgr);
-  debug::Alias(&num_families);
+  WTF::debug::Alias(&font_cache);
+  WTF::debug::Alias(&font_mgr);
+  WTF::debug::Alias(&num_families);
 
   CHECK(false);
 }

third_party/WebKit/Source/platform/fonts/UTF16TextIterator.cpp

@@ -23,9 +23,6 @@
 
 #include "platform/fonts/UTF16TextIterator.h"
 
-using namespace WTF;
-using namespace Unicode;
-
 namespace blink {
 
 UTF16TextIterator::UTF16TextIterator(const UChar* characters, int length)
@@ -56,7 +53,7 @@ bool UTF16TextIterator::ConsumeSurrogatePair(UChar32& character) {
   DCHECK(U16_IS_SURROGATE(character));
 
   if (!IsValidSurrogatePair(character)) {
-    character = kReplacementCharacter;
+    character = WTF::Unicode::kReplacementCharacter;
     return true;
   }