Make rebase_path() aware of Windows drive letter capitalization

tools/gn/filesystem_utils_unittest.cc

Index: tools/gn/filesystem_utils_unittest.cc
diff --git a/tools/gn/filesystem_utils_unittest.cc b/tools/gn/filesystem_utils_unittest.cc
index d5638900e2e204802df6acb9cac974cc87194fb6..894b3da7d2393cc2549bd6ef020e34c7fa23a31d 100644
--- a/tools/gn/filesystem_utils_unittest.cc
+++ b/tools/gn/filesystem_utils_unittest.cc
@@ -488,6 +488,24 @@ TEST(FilesystemUtils, RebasePath) {
             RebasePath("/path/to/foo", SourceDir("/source/root/a/b"),
                        base::StringPiece("/x/y/z")));
 
+#if defined(OS_WIN)
+  // Test corrections while rebasing Windows-style absolute paths.
+  EXPECT_EQ("../../../../path/to/foo",
+            RebasePath("C:/path/to/foo", SourceDir("//a/b"),
+                       base::StringPiece("/C:/source/root")));
+  EXPECT_EQ("../../../../path/to/foo",
+            RebasePath("/C:/path/to/foo", SourceDir("//a/b"),
+                       base::StringPiece("C:/source/root")));
+  EXPECT_EQ("../../../../path/to/foo",
+            RebasePath("/C:/path/to/foo", SourceDir("//a/b"),
+                       base::StringPiece("/c:/source/root")));
+  EXPECT_EQ("../../../../path/to/foo",
+            RebasePath("/c:/path/to/foo", SourceDir("//a/b"),
+                       base::StringPiece("c:/source/root")));
+  EXPECT_EQ("../../../../path/to/foo",
+            RebasePath("/c:/path/to/foo", SourceDir("//a/b"),
+                       base::StringPiece("C:/source/root")));
+#endif
 }
 
 TEST(FilesystemUtils, DirectoryWithNoLastSlash) {