Decouple SourceType enum from net::SourceStream class

This CL extracts the SourceStream::SourceType enum into a new
SourceStreamType enum class, defined in source_stream_type.h.

This change addresses the issue where users needing the SourceType enum
were forced to include source_stream.h, even if they had no other
dependencies on the SourceStream class.

By creating a dedicated header for SourceStreamType, we improve
modularity and reduce unnecessary header dependencies.

This CL must not have any behavioral changes.

NO_IFTTT=New LINT.If/ThenChange()

Bug: 391950057
Change-Id: Ia25db924a0e7a8d1b379dd1042ede0e898cd6dd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6278220
Reviewed-by: Takashi Toyoshima <[email protected]>
Reviewed-by: John Lee <[email protected]>
Reviewed-by: Dominic Battré <[email protected]>
Reviewed-by: Rakina Zata Amni <[email protected]>
Commit-Queue: Tsuyoshi Horo <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Gauthier Ambard <[email protected]>
Reviewed-by: Lei Zhang <[email protected]>
Reviewed-by: Benjamin Gordon <[email protected]>
Reviewed-by: Nidhi Jaju <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1422841}
diff --git a/ui/base/webui/i18n_source_stream_unittest.cc b/ui/base/webui/i18n_source_stream_unittest.cc
index 01f25a5..d650ef8 100644
--- a/ui/base/webui/i18n_source_stream_unittest.cc
+++ b/ui/base/webui/i18n_source_stream_unittest.cc
@@ -102,7 +102,7 @@
     replacements_["beta"] = "banana";
     replacements_["gamma"] = "carrot";
     stream_ = I18nSourceStream::Create(
-        std::move(source), net::SourceStream::TYPE_NONE, &replacements_);
+        std::move(source), net::SourceStreamType::kNone, &replacements_);
   }
 
   // If MockSourceStream::Mode is ASYNC, completes 1 read from |mock_stream| and