Revert of mac and ios: Build third-party code with -Wall. (patchset #1 id:1 of https://codereview.chromium.org/1555843002/ )

Reason for revert:
ios_Device builder isn't quite ready for -Wall,

 http://build.chromium.org/p/chromium.mac/builders/iOS_Device/builds/33368

breaking compilation.

Original issue's description:
> mac and ios: Build third-party code with -Wall.
>
> This lands the mac and ios build/common.gypi bits of
> https://codereview.chromium.org/1551753002/ (reviewed there)
>
> BUG=573250
> R=thestig@chromium.org
> TBR=thestig@chromium.org
>
> Committed: https://crrev.com/9830789346abd3d8211deff1ebe7a7f5753ba3fc
> Cr-Commit-Position: refs/heads/master@{#367255}

TBR=thestig@chromium.org,thakis@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=573250

Review URL: https://codereview.chromium.org/1553943002

Cr-Original-Commit-Position: refs/heads/master@{#367258}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 02b240dbc9219e8ae755dd99ded199ac7b4bf77f

common.gypi

@@ -3206,7 +3206,7 @@
 
           [ 'OS=="mac" or OS=="ios"', {
             'xcode_settings': {
-              'WARNING_CFLAGS!': ['-Wextra'],
+              'WARNING_CFLAGS!': ['-Wall', '-Wextra'],
             },
             'conditions': [
               ['buildtype=="Official"', {