[aw] Don't allow null host string in setProxyOverride

Null Java strings shouldn't be passed down to C++ code. This CL adds
code to check for a null host string or null strings in exclusionList
and throw a null pointer exception.

Bug: 896285
Change-Id: I6af64350de8a93031741c8951a7efe2df7111ccd
Reviewed-on: https://chromium-review.googlesource.com/c/1286849Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Laís Minchillo <laisminchillo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600731}

android_webview/glue/java/src/com/android/webview/chromium/SharedStatics.java

@@ -119,6 +119,16 @@ public class SharedStatics {
     }
 
     public void setProxyOverride(String host, int port, String[] exclusionList, Runnable callback) {
+        if (host == null) {
+            throw new NullPointerException("Host string should not be null");
+        }
+        if (exclusionList != null) {
+            for (String url : exclusionList) {
+                if (url == null) {
+                    throw new NullPointerException("Excluded URL strings should not be null");
+                }
+            }
+        }
         ThreadUtils.runOnUiThread(
                 () -> AwContentsStatics.setProxyOverride(host, port, exclusionList, callback));
     }