devtools_protocol.cc: fix ptr to bool conversion warning

devtools_protocol.cc: fix ptr to bool conversion warning

../../chrome/browser/devtools/devtools_protocol.cc:120:12: error: implicit conversion of nullptr constant to 'bool' [-Werror,-Wnull-conversion]
    return nullptr;
    ~~~~~~ ^~~~~~~
           false

BUG=431803
TBR=pfeldman
NOTRY=true

Committed: https://crrev.com/448c20706e3786c10db5abeadd2ada9c20a8ac81
Cr-Commit-Position: refs/heads/master@{#307389}

[hans, 2014-12-09 00:54:51]

hans@chromium.org changed reviewers:
+ pfeldman@chromium.org

[hans, 2014-12-09 00:54:55]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2014-12-09 00:55:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/786903002/1

[commit-bot: I haz the power, 2014-12-09 00:56:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-09 00:57:18]

Patchset 1 (id:??) landed as
https://crrev.com/448c20706e3786c10db5abeadd2ada9c20a8ac81
Cr-Commit-Position: refs/heads/master@{#307389}