Enable -Wenum-enum-conversion in blink (which enables -Wconversion).

Looks like all instances of the warning are fixed now.

Bug: 1035171
Change-Id: I1a790f9c483525a8b72bda70f7dc27950321bfdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031683
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737352}

third_party/blink/renderer/BUILD.gn

@@ -50,10 +50,7 @@ config("inside_blink") {
     ]
 
     if (!is_chromeos || default_toolchain != "//build/toolchain/cros:target") {
-      cflags += [
-        "-Wno-enum-enum-conversion",
-        "-Wno-enum-float-conversion",
-      ]
+      cflags += [ "-Wno-enum-float-conversion" ]
     }
   }
 }