Removed Webview keeps from main dex rules

Since Webview is only shipped on L+, it shouldn't matter which dex file webview
is found in. These keeps prevent optimizations happening across the main dex
boundary, so it is preferred to remove.

Bug: 887942
Change-Id: I1b40c4537b0d5246ec4b3de3f622637fe688900b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598316
Auto-Submit: Sam Maier <smaier@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657264}

build/android/main_dex_classes.flags

@@ -29,15 +29,6 @@
   *;
 }
 
-# WebView doesn't load secondary dex.
--keep public class com.android.webview.** {
-  *;
-}
-
--keep public class org.chromium.android_webview.** {
-  *;
-}
-
 # Used by tests for secondary dex extraction.
 -keep class android.support.v4.content.ContextCompat {
   *;