Fail to build with unbound Info.plist substitutions instead of silently dropping them.

extensions/shell/app/app-Info.plist

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
         <key>CFBundleDevelopmentRegion</key>
         <string>en</string>
         <key>CFBundleDisplayName</key>
         <string>${EXECUTABLE_NAME}</string>
         <key>CFBundleExecutable</key>
         <string>${EXECUTABLE_NAME}</string>
         <key>CFBundleIconFile</key>
         <string>app.icns</string>
         <key>CFBundleIdentifier</key>
         <string>org.chromium.AppShell</string>
         <key>CFBundleInfoDictionaryVersion</key>
         <string>6.0</string>
         <key>CFBundleName</key>
         <string>${PRODUCT_NAME}</string>
         <key>CFBundlePackageType</key>
         <string>APPL</string>
         <key>CFBundleVersion</key>
         <string>1.0</string>
         <key>NSMainNibFile</key>
         <string>MainMenu</string>
         <key>NSPrincipalClass</key>
         <string>NSApplication</string>
         <key>LSMinimumSystemVersion</key>
-        <string>${MACOSX_DEPLOYMENT_TARGET}.0</string>
+        <string>10.9.0</string>

[sdefresne, 2017/06/27 08:26:25]

I don't like hardcoding the value here. It is available in GN via the
"mac_deployment_target" variable from build/config/mac/mac_sdk.gni, can't we
instead add the following to the template that does variable substitution on
.plist files:

  MACOSX_DEPLOYMENT_TARGET=$mac_deployment_target

This will avoid requiring to grep all files for "10.9" when this variable is
updated.

[Sidney San Martín, 2017/06/27 19:45:50]

Done. I just remembered why it was like this in the original CL: at that point,
our minimum supported macOS version was 10.9, but our build deployment target
was 10.7, and I was discouraged from adding a separate build variable that would
just be used in Info.plists. Now the target is back in sync :) (crbug/580152).

         <key>LSFileQuarantineEnabled</key>
         <true/>
         <key>NSSupportsAutomaticGraphicsSwitching</key>
         <true/>
 </dict>
 </plist>