Bug 1541095: Remove an unnecessary static_cast in WritingModes.h. r=TYLin

The variables involved in the bitwise arithmetic are all of type uint8_t, so
we end up with that type automatically (no need to bother with a static_cast).

Note that we *do* need to declare the type (we can't use `auto`, or else
the compiler wants to upgrade the local variable to have a wider type --
"int", I think -- and that then produces an build warning when we pass
that variable as a uint8_t param further down).

Differential Revision: https://phabricator.services.mozilla.com/D61893

--HG--
extra : moz-landing-system : lando

layout/generic/WritingModes.h

@@ -447,9 +447,9 @@ class WritingMode {
       MOZ_ASSERT(StyleWritingMode::VERTICAL.bits == 0x01 &&
                      StyleWritingMode::VERTICAL_LR.bits == 0x04,
                  "unexpected mask values");
-      const auto wm = static_cast<uint8_t>(
+      const uint8_t wm =
           ((mWritingMode & StyleWritingMode::VERTICAL_LR).bits >> 1) |
-          (mWritingMode & StyleWritingMode::VERTICAL).bits);
+          (mWritingMode & StyleWritingMode::VERTICAL).bits;
       return PhysicalSideForBlockAxis(wm, GetEdge(aSide));
     }