[Android WebView] Enable disk access in data dir locking code

Dir locking code needs to run synchronously before starting
the browser process, as if we figure out that the dir is
already in use we need to avoid starting the browser, as
it will mess up the data.

Thus the only way to alleviate strict mode violation is to
enable disk access.

BUG=558377,593353

Review URL: https://codereview.chromium.org/1781653004

Cr-Commit-Position: refs/heads/master@{#380738}
(cherry picked from commit c092dc5)

Review URL: https://codereview.chromium.org/1786273002 .

Cr-Commit-Position: refs/branch-heads/2661@{crosswalk-project#199}
Cr-Branched-From: ef6f6ae-refs/heads/master@{#378081}

android_webview/java/src/org/chromium/android_webview/AwBrowserProcess.java

@@ -5,6 +5,7 @@
 package org.chromium.android_webview;
 
 import android.content.Context;
+import android.os.StrictMode;
 
 import org.chromium.android_webview.policy.AwPolicyProvider;
 import org.chromium.base.CommandLine;
@@ -93,20 +94,26 @@ public void run() {
     }
 
     private static void tryObtainingDataDirLockOrDie(Context context) {
-        String dataPath = PathUtils.getDataDirectory(context);
-        File lockFile = new File(dataPath, EXCLUSIVE_LOCK_FILE);
-        boolean success = false;
+        StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
+        StrictMode.allowThreadDiskWrites();
         try {
-            // Note that the file is not closed intentionally.
-            RandomAccessFile file = new RandomAccessFile(lockFile, "rw");
-            sExclusiveFileLock = file.getChannel().tryLock();
-            success = sExclusiveFileLock != null;
-        } catch (IOException e) {
-            Log.w(TAG, "Failed to create lock file " + lockFile, e);
-        }
-        if (!success) {
-            Log.w(TAG, "The app may have another WebView opened in a separate process. "
-                    + "This is not recommended and may stop working in future versions.");
+            String dataPath = PathUtils.getDataDirectory(context);
+            File lockFile = new File(dataPath, EXCLUSIVE_LOCK_FILE);
+            boolean success = false;
+            try {
+                // Note that the file is not closed intentionally.
+                RandomAccessFile file = new RandomAccessFile(lockFile, "rw");
+                sExclusiveFileLock = file.getChannel().tryLock();
+                success = sExclusiveFileLock != null;
+            } catch (IOException e) {
+                Log.w(TAG, "Failed to create lock file " + lockFile, e);
+            }
+            if (!success) {
+                Log.w(TAG, "The app may have another WebView opened in a separate process. "
+                        + "This is not recommended and may stop working in future versions.");
+            }
+        } finally {
+            StrictMode.setThreadPolicy(oldPolicy);
         }
     }
 }