Reland "android: create cache dir if needed."

Remove the accidentally committed assert that caused test failures and
reland:

> When using a subdirectory of the Android cache dir (e.g. in WebView),
> explicitly create the directory in PathUtils instead of relying on any
> user of the directory creating it implicitly.

> Set the directory mode to 02700 which is what it currently gets set to
> when created implicitly.

This reverts commit fbdad03f.

Fixed: 963640
Fixed: 1132987
Change-Id: Ic94acaa1fd4dacb55839086699cc2974496e6bce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438469Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811785}

base/android/java/src/org/chromium/base/PathUtils.java

@@ -108,8 +108,11 @@ public abstract class PathUtils {
             if (sCacheSubDirectory == null) {
                 paths[CACHE_DIRECTORY] = appContext.getCacheDir().getPath();
             } else {
-                paths[CACHE_DIRECTORY] =
-                        new File(appContext.getCacheDir(), sCacheSubDirectory).getPath();
+                File cacheDir = new File(appContext.getCacheDir(), sCacheSubDirectory);
+                cacheDir.mkdir();
+                paths[CACHE_DIRECTORY] = cacheDir.getPath();
+                // Set to rwx--S--- as the Android cache dir has a distinct gid and is setgid.
+                chmod(paths[CACHE_DIRECTORY], 02700);
             }
         }
         return paths;