Add ifdef guard to WorkletModuleTreeClient.h

We use "jumbo" compilation of blink in our project. The main idea is
to compile some number of cc files as one compilation unit. It
significantly speeds up compilation time of blink. With "jumbo"
enabled there is compilation error due to lack of ifdef
guard in file WorkletModuleTreeClient.h.

BUG=732727

Review-Url: https://codereview.chromium.org/2935823003
Cr-Commit-Position: refs/heads/master@{#478967}
1 file changed