Reland "Use in-class initializers for base::WeakPtrFactory in base/."

This is a reland of 66ec904fa26ddc098d0941277e162cc9de5cb8ce

Original change's description:
> Use in-class initializers for base::WeakPtrFactory in base/.
> 
> Generated by: https://chromium-review.googlesource.com/c/chromium/src/+/1689216
> 
> Bug: 981415
> Change-Id: Ib185d9d46bbfa787b77fbbafcc3e5bfb7799dd20
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688886
> Reviewed-by: Daniel Cheng <[email protected]>
> Commit-Queue: Jeremy Roman <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#674950}

Bug: 981415
Change-Id: I197ea3bbe5170488e7796319bc60bad5da0ed5ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688608
Auto-Submit: Jeremy Roman <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#675035}
diff --git a/base/files/important_file_writer.h b/base/files/important_file_writer.h
index af919090..d3a42ef 100644
--- a/base/files/important_file_writer.h
+++ b/base/files/important_file_writer.h
@@ -151,7 +151,7 @@
 
   SEQUENCE_CHECKER(sequence_checker_);
 
-  WeakPtrFactory<ImportantFileWriter> weak_factory_;
+  WeakPtrFactory<ImportantFileWriter> weak_factory_{this};
 
   DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
 };