split off from  https://crbug.com/857121#c12 

found in 69.0.3497.100

D StrictMode: StrictMode policy violation; ~duration=78 ms: android.os.strictmode.DiskReadViolation
D StrictMode:   at android.os.StrictMode$AndroidBlockGuardPolicy.onReadFromDisk(StrictMode.java:1504)
D StrictMode:   at java.io.UnixFileSystem.checkAccess(UnixFileSystem.java:251)
D StrictMode:   at java.io.File.exists(File.java:815)
D StrictMode:   at android.app.ContextImpl.ensurePrivateDirExists(ContextImpl.java:606)
D StrictMode:   at android.app.ContextImpl.ensurePrivateDirExists(ContextImpl.java:597)
D StrictMode:   at android.app.ContextImpl.getPreferencesDir(ContextImpl.java:553)
D StrictMode:   at android.app.ContextImpl.getSharedPreferencesPath(ContextImpl.java:748)
D StrictMode:   at android.app.ContextImpl.getSharedPreferences(ContextImpl.java:401)
D StrictMode:   at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:174)
D StrictMode:   at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:174)
D StrictMode:   at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:526)
D StrictMode:   at akb.<clinit>(SourceFile:1)
D StrictMode:   at org.chromium.components.variations.firstrun.VariationsSeedBridge.markVariationsSeedAsStored(SourceFile:23)
D StrictMode:   at org.chromium.content.browser.BrowserStartupControllerImpl.nativeFlushStartupTasks(Native Method)
D StrictMode:   at org.chromium.content.browser.BrowserStartupControllerImpl.a(SourceFile:50)
D StrictMode:   at afP.run(Unknown Source:36)
D StrictMode:   at org.chromium.base.ThreadUtils.a(SourceFile:10)
D StrictMode:   at xf.b(SourceFile:107)
D StrictMode:   at xf.b(SourceFile:172)
D StrictMode:   at xf.a(SourceFile:153)
D StrictMode:   at com.android.webview.chromium.WebViewChromiumFactoryProvider.a(SourceFile:231)
D StrictMode:   at com.android.webview.chromium.WebViewChromium.init(SourceFile:44)
D StrictMode:   at android.webkit.WebView.<init>(WebView.java:678)
D StrictMode:   at android.webkit.WebView.<init>(WebView.java:604)
D StrictMode:   at android.webkit.WebView.<init>(WebView.java:587)
D StrictMode:   at android.webkit.WebView.<init>(WebView.java:574)
D StrictMode:   at android.webkit.WebView.<init>(WebView.java:564)
D StrictMode:   at com.android.cts.webkit.WebViewStartupCtsActivity.createAndAttachWebView(WebViewStartupCtsActivity.java:33)
D StrictMode:   at com.android.cts.webkit.WebViewDeviceSideStartupTest.createWebViewAndNavigate(WebViewDeviceSideStartupTest.java:198)
D StrictMode:   at com.android.cts.webkit.WebViewDeviceSideStartupTest.testStrictModeNotViolatedOnStartup(WebViewDeviceSideStartupTest.java:183)

WebView no longer invokes VariationsSeedBridge.setVariationsFirstRunSeed() as of https://crrev.com/c/1081305, but VariationsSeedStore::StoreSeedDataNoDelta() still invokes VariationsSeedBridge.markVariationsSeedAsStored(). This uses does unnecessary file I/O.