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This is a reland of 2903236d

Original change's description:
> Change NavigationListModel to support MyFiles as Volume.
> 
> Add MYFILES_VOLUME_ENABLED in private strings to be able to have the
> feature flag in the JS/UI code.
> 
> Change VolumeManager to use MyFiles folder for Downloads volume, the
> method |GetMyFilesFolderForProfile| takes care of returning the right
> value based on the feature flag MyFilesVolume.
> 
> Change NavigationListModel to support MyFiles as Volume when
> MyFilesVolume feature flag is enabled:
> 1. MyFiles volume is the Downloads volume (after the feature flag is
> removed we can rename it accordingly). Setup Downloads volume as
> VolumeEntry,
> 2. Create a NavigationModelFakeItem with "My files" label, based on the
> VolumeEntry (step #1).
> 3. Skip adding "Downloads" to Myfiles, since now MyFiles is in fact
> Downloads volume.
> 4. Continue to add Linux and Play files to Myfiles.
> 
> Test: unit_tests --gtest_filter='VolumeManagerTest.GetVolumeListMyFilesVolume' and browser_test --gtest_filter='FileManagerJsTest.DirectoryTreeTest:FileManagerJsTest.NavigationList*'
> Bug: 873539
> Change-Id: Ia242d52a1e4d7b4fb3c7ca219d9cfdc4fce72543
> Reviewed-on: https://chromium-review.googlesource.com/c/1308235
> Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
> Reviewed-by: Joel Hockey <joelhockey@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#604143}

Bug: 873539
Change-Id: I396cda87781c21856a736c6f5eae8f76efb4e62c
Reviewed-on: https://chromium-review.googlesource.com/c/1314012Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604836}