Add proguard definitions to prevent stripping of methods
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379922}

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