Improve support for reading android virtual content uri as FilePath

go/clank-files-clipboard-drag-drop

When chrome does drag-drop of an image out of a renderer, it saves the
image in memory as a virtual content-uri.  This CL improves support for
accessing the virtual content-uri.

The DropDataProvider adds implementation of ContentProvider#openAssetFile()
which returns an AssetFileDescriptor with the valid length.

ContentUriUtils.getAssetFileDescriptor() is fixed to use
ContentResolver#openAssetFileDescriptor() which will include the valid
length, and adds a utility function getContentUriFileSize() which is used by file_util_posix.cc if fstat returns size=0.

LocalFileStreamReader which is used when a renderer fetches the blob
contents of a dropped file is changed to avoid calling seek() when
initial offset is zero, and seek is not required.  Any call to seek()
fails on android for a virtual file.

Bug: b/350860794
Low-Coverage-Reason: ContentUriUtils.java changes tested manually
Change-Id: I9842d428c62ecbb3b9542dcd1a293b31791dbdf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5679910
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Shu Yang <shuyng@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1324600}

• base/android/content_uri_utils.cc[diff]
• base/android/content_uri_utils.h[diff]
• base/android/java/src/org/chromium/base/ContentUriUtils.java[diff]
• base/files/file_util_posix.cc[diff]
• storage/browser/file_system/local_file_stream_reader.cc[diff]
• ui/android/java/src/org/chromium/ui/dragdrop/DropDataContentProvider.java[diff]
• ui/android/java/src/org/chromium/ui/dragdrop/DropDataProviderImpl.java[diff]