Disable Quick View by default and add about flag to enable it.

Set --enable-files-quick-view to enable quick view.

BUG=613465
TEST=manually using link.

Review-Url: https://codereview.chromium.org/2127853002
Cr-Commit-Position: refs/heads/master@{#404617}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index c6c6ccb..62686c60 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1968,6 +1968,10 @@
     {"enable-intent-picker", IDS_ENABLE_INTENT_PICKER,
      IDS_ENABLE_INTENT_PICKER_DESCRIPTION, kOsCrOS,
      SINGLE_VALUE_TYPE(switches::kEnableIntentPicker)},
+    {"files-quick-view", IDS_FLAGS_FILES_QUICK_VIEW_NAME,
+     IDS_FLAGS_FILES_QUICK_VIEW_DESCRIPTION, kOsCrOS,
+     ENABLE_DISABLE_VALUE_TYPE(chromeos::switches::kEnableFilesQuickView,
+                               chromeos::switches::kDisableFilesQuickView)},
 #endif  // defined(OS_CHROMEOS)
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in