Adding `content_security_policy` to proxy sample extension.

chrome/common/extensions/docs/examples/extensions/proxy_configuration/popup.css

+body {
+  margin: 5px 10px 10px;
+}
+
+h1 {
+  color: #53637D;
+  font: 26px/1.2 Helvetica, sans-serif;
+  font-size: 200%;
+  margin: 0;
+  padding-bottom: 4px;
+  text-shadow: white 0 1px 2px;
+}
+
+div[role='main'] {
+  border-radius: 5px;
+  background: #EAEEF3;
+  font: 14px/1 Arial,Sans Serif;
+  padding: 10px;
+  width: 563px;
+  box-shadow: inset 0px 2px 5px rgba(0,0,0,0.5);
+  -webkit-transition: background-color 0.5s ease-out;
+  overflow: hidden;
+}
+
+div[role='main'].incognito {
+  background: #496281 url('data:image/png;base64,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') no-repeat 533px bottom;
+}
+
+form {
+  width: 563px;
+  -webkit-transition: -webkit-transform 0.25s ease;
+}
+
+form.offscreen {
+  -webkit-transform: translateX(-600px);
+}
+
+fieldset {
+  border: 0;
+  margin: 0;
+  padding: 0;
+  position: relative;
+}
+
+legend {
+  position: absolute;
+  left: -999em;
+}
+
+form > fieldset {
+  border-radius:  5px;
+  border: 1px solid transparent;
+  padding: 10px 10px 10px 30px;
+  margin: 5px 0;
+  -webkit-transition: all 0.5s ease;
+}
+
+form > fieldset:hover {
+  background: rgba(255,255,255,0.1);
+  border-color: rgba(0,0,0,0.1);
+}
+
+form > fieldset.active {
+  background: rgba(255,255,255,0.25);
+  border-color: rgba(0,0,0,0.25);
+}
+
+form > fieldset > input {
+  margin-left:  -20px;
+}
+
+section {
+  margin: 5px 0 0;
+}
+
+section fieldset:not(:first-child):not(:last-child) {
+  -webkit-transition: all 0.5s ease;
+  overflow: hidden;
+  max-height: 1.6em;
+}
+
+section.single fieldset:not(:first-child):not(:last-child) {
+  max-height: 0px;
+}
+
+section fieldset:last-child {
+  margin-top: 5px;
+}
+
+section fieldset:last-child label {
+  display: block;
+}
+
+section fieldset:last-child textarea {
+  width: 412px;
+}
+
+section > fieldset {
+  position: relative;
+  padding-left: 60px;
+}
+
+section > fieldset > legend {
+  left: 0;
+  top: 4px;
+  width: 53px;
+  text-align: right;
+}
+
+input[type='url']:invalid:not(:active):not(:focus) {
+  border-color: rgba(255,0,0,0.5);
+  background: rgba(255,0,0,0.25);
+}
+
+input:invalid:not(:active):not(:focus):after {
+  content: "This isn't a valid URL!";
+  display:block;
+}
+
+input[type="checkbox"] {
+  margin: 5px 0 5px 35px;
+}
+
+input[type="text"] {
+  width: 200px;
+  margin: 0 10px 0 0;
+}
+
+input.port {
+  width: 50px;
+  margin: 2px 10px 0 5px;
+}
+
+section label,
+section legend {
+  color:  #999;
+  -webkit-transition: color 0.5s ease;
+}
+
+.incognito section label,
+.incognito section legend {
+  color:  #BBB;
+}
+
+.active section label,
+.active section legend,
+form > fieldset > label  {
+  color:  #000;
+  -webkit-transition: color 0.5s ease;
+}
+
+.incognito .active section label,
+.incognito .active section legend,
+.incognito form > fieldset > label {
+  color: #FFF;
+}
+
+input[type="submit"],
+button {
+  border-radius: 2px;
+  box-shadow: 0px 1px 3px rgba(0, 0, 0, 0.1);
+  -webkit-user-select: none;
+  background: -webkit-linear-gradient(#FAFAFA, #F4F4F4 40%, #E5E5E5);
+  border: 1px solid #AAA;
+  color: #444;
+  margin-bottom: 0;
+  min-width: 4em;
+  padding: 3px 12px;
+  margin-top: 0;
+  font-size: 1.1em;
+}
+
+.overlay {
+  display: block;
+  text-align: center;
+  position: absolute;
+  left: 50%;
+  top: 50%;
+  width: 500px;
+  padding: 20px;
+  margin: -80px 0 0 -270px;
+  opacity: 0;
+  background: rgba(0, 0, 0, 0.75);
+  border-radius: 5px;
+  color: #FFF;
+  font: 1.5em/1.2 Helvetica Neue, sans-serif;
+  -webkit-transition: all 1.0s ease;
+  -webkit-transform: scale(0);
+}
+
+.overlay a {
+  color:  #FFF;
+}
+
+.overlay.visible {
+  opacity: 1;
+  -webkit-transform: scale(1);
+}