Revert of Revert of Clang on Windows: Remove -Wno-unused-variable. (patchset #1 id:1 of https://codereview.chromium.org/1228093005/)

Reason for revert:
might be better now?

Original issue's description:
> Revert of Clang on Windows: Remove -Wno-unused-variable. (patchset #3 id:40001 of https://codereview.chromium.org/1226573002/)
>
> Reason for revert:
> Broke Clang build on Windows according to http://crbug.com/505319#c30.
>
> Those warnings should be fixed before relanding.
>
> Original issue's description:
> > Clang on Windows: Remove -Wno-unused-variable.
> >
> > Unused variables are now errors. However, since lots of third-party code
> > has unused variables, added a blanket suppression to all third-party
> > code.
> >
> > BUG=505319
> >
> > Committed: https://crrev.com/3371ccbd9d83e1031c5891520e88acbc9c1859a0
> > Cr-Commit-Position: refs/heads/master@{#338242}
>
> TBR=garykac@chromium.org,thakis@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=505319
>
> Committed: https://crrev.com/ff68a3ba6f2dc9b226047b3355c135f73352570e
> Cr-Commit-Position: refs/heads/master@{#338253}

TBR=garykac@chromium.org,mgiuca@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=505319

Review URL: https://codereview.chromium.org/1232543005

Cr-Original-Commit-Position: refs/heads/master@{#338395}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5a503569600d998528568b06c297cbe2d0e9bcb4

common.gypi

@@ -3111,6 +3111,9 @@
             # TODO(mgiuca): Move this suppression into individual third-party
             # libraries as required. http://crbug.com/505301.
             '-Wno-overloaded-virtual',
+            # Lots of third-party libraries have unused variables. Instead of
+            # suppressing them individually, we just blanket suppress them here.
+            '-Wno-unused-variable',
           ],
         },
         'conditions': [
@@ -5733,7 +5736,6 @@
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-unused-function',  # http://crbug.com/505316
                   '-Wno-unused-value',  # http://crbug.com/505318
-                  '-Wno-unused-variable',  # http://crbug.com/505319
                   '-Wno-unused-local-typedef',  # http://crbug.com/411648
                 ],
               },

config/compiler/BUILD.gn

@@ -806,7 +806,6 @@ if (is_win) {
       "-Wno-unknown-pragmas",  # http://crbug.com/505314
       "-Wno-unused-function",  # http://crbug.com/505316
       "-Wno-unused-value",  # http://crbug.com/505318
-      "-Wno-unused-variable",  # http://crbug.com/505319
       "-Wno-unused-local-typedef",  # http://crbug.com/411648
     ]
   }
@@ -1003,6 +1002,10 @@ config("no_chromium_code") {
       # TODO(mgiuca): Move this suppression into individual third-party
       # libraries as required. http://crbug.com/505301.
       "-Wno-overloaded-virtual",
+
+      # Lots of third-party libraries have unused variables. Instead of
+      # suppressing them individually, we just blanket suppress them here.
+      "-Wno-unused-variable",
     ]
   }