Add proguard definitions to prevent stripping of methods

Newly added WebviewChromium methods getHandler and findFocus
were being stripped by proguard. Explicitly keep them in .jar.

Committed: https://crrev.com/4e0e6528676be9ee35c31286c19f7bb9d4c7e957
Cr-Commit-Position: refs/heads/master@{#379922}
  

Merge approved for M50 branch 2661.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c8a016c402bcc923349a3c840bcb196bcfc1425d

commit c8a016c402bcc923349a3c840bcb196bcfc1425d
Author: Selim Gurun <sgurun@google.com>
Date: Sat Mar 12 00:40:55 2016

Add proguard definitions to prevent stripping of methods

Newly added WebviewChromium methods getHandler and findFocus
were being stripped by proguard. Explicitly keep them in .jar.

Review URL: https://codereview.chromium.org/1765333002

Cr-Commit-Position: refs/heads/master@{#379922}
(cherry picked from commit 4e0e6528676be9ee35c31286c19f7bb9d4c7e957)

BUG= 594274 

Review URL: https://codereview.chromium.org/1787733004 .

Cr-Commit-Position: refs/branch-heads/2661@{#201}
Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}

[modify] https://crrev.com/c8a016c402bcc923349a3c840bcb196bcfc1425d/android_webview/apk/java/proguard.flags