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This allows us to control the final binary size by optionally including features.

We only want clients to import one umbrella header:
  #import <ChromeWebView/ChromeWebView.h>
To accomplish this, we generate the umbrella header based on the feature flags.

Binary size for Release-iphoneos with Cronet included:
+signin and +autofill: 20MB
-signin and -autofill: 15.5MB
+signin and -autofill: 16.7MB
-signin and +autofill: 19.6MB

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2981b06812495d9b24b1d9630f010247a7ab1331
Reviewed-on: https://chromium-review.googlesource.com/980737
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549276}