Enable -Wcharacter-conversion

Now that the pdfium fix has rolled in, we should be able to build with
this warning enabled.

Bug: 417996267
Change-Id: I986a55739cefb4bfc9744ca60613c71d9e4292d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6674713
Reviewed-by: Hans Wennborg <[email protected]>
Commit-Queue: Devon Loehr <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1479782}
NOKEYCHECK=True
GitOrigin-RevId: f1cdd12cc7a8b10cf24f3442c45e9e72565eb7bb

Author: DKLoehr

config/compiler/BUILD.gn

@@ -2107,12 +2107,6 @@ config("default_warnings") {
       ]
     }
 
-    if (default_toolchain != "//build/toolchain/cros:target" &&
-        !llvm_android_mainline) {
-      # TODO(crbug.com/417996267): Fix and re-enable
-      cflags += [ "-Wno-character-conversion" ]
-    }
-
     if (is_component_build && use_clang_warning_suppression_file && !is_win) {
       # Warn for globally unique objects which might get duplicated if built
       # into different components, due to hidden visibility. Only applicable