Start a market Intent with Intent.FLAG_ACTIVITY_NEW_TASK

In document mode, the current task could be removed after a market Intent
is started.
So, we should start it in a new task.

BUG=487295

Review URL: https://codereview.chromium.org/1149863003

Cr-Commit-Position: refs/heads/master@{#330836}

chrome/android/java/src/org/chromium/chrome/browser/externalnav/ExternalNavigationHandler.java

@@ -247,6 +247,7 @@ public class ExternalNavigationHandler {
                             + "&referrer=" + mDelegate.getPackageName()));
                     intent.addCategory(Intent.CATEGORY_BROWSABLE);
                     intent.setPackage("com.android.vending");
+                    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                     mDelegate.startActivity(intent);
                     return OverrideUrlLoadingResult.OVERRIDE_WITH_EXTERNAL_INTENT;
                 } catch (ActivityNotFoundException ex) {