Fix a findbugs warning - check file.mkdirs return value.

BUG=NONE

Review URL: https://codereview.chromium.org/262743003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@268018 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/findbugs_filter/findbugs_known_bugs.txt

@@ -25,4 +25,3 @@ M V EI: org.chromium.chrome.browser.ChromeBrowserProvider$BookmarkNode.thumbnail
 M V EI: org.chromium.content.browser.LoadUrlParams.getPostData() may expose internal representation by returning LoadUrlParams.mPostData  At LoadUrlParams.java
 M M LI: Incorrect lazy initialization of static field org.chromium.chrome.browser.sync.ProfileSyncService.sSyncSetupManager in org.chromium.chrome.browser.sync.ProfileSyncService.get(Context)  At ProfileSyncService.java
 M B OS: org.chromium.tools.binary_size.Addr2LineWorkerPool$Addr2LineWorker$Addr2LineTask.run() may fail to close stream  At Addr2LineWorkerPool.java
-M B RV: Exceptional return value of java.io.File.mkdirs() ignored in new org.chromium.tools.binary_size.NmDumper$Output(NmDumper)  At NmDumper.java